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# std.exception

This module defines functions related to exceptions and general error handling. It also defines functions intended to aid in unit testing.

Category | Functions
--- | ---
Assumptions | assertNotThrown assertThrown assumeUnique assumeWontThrow mayPointTo
Enforce | doesPointTo enforce errnoEnforce
Handlers | collectException collectExceptionMsg ifThrown handle
Other | basicExceptionCtors emptyExceptionMsg ErrnoException RangePrimitive

License:
Boost License 1.0
Authors:
Andrei Alexandrescu and Jonathan M Davis

Source std/exception.d

Examples:
Synopis

    import core.stdc.stdlib : malloc, free;
    import std.algorithm.comparison : equal;
    import std.algorithm.iteration : map, splitter;
    import std.algorithm.searching : endsWith;
    import std.conv : ConvException, to;
    import std.range : front, retro;

    // use enforce like assert
    int a = 3;
    enforce(a > 2, "a needs to be higher than 2.");

    // enforce can throw a custom exception
    enforce!ConvException(a > 2, "a needs to be higher than 2.");

    // enforce will return it's input
    enum size = 42;
    auto memory = enforce(malloc(size), "malloc failed")[0 .. size];
    scope(exit) free(memory.ptr);

    // collectException can be used to test for exceptions
    Exception e = collectException("abc".to!int);
    assert(e.file.endsWith("conv.d"));

    // and just for the exception message
    string msg = collectExceptionMsg("abc".to!int);
    writeln(msg); // "Unexpected 'a' when converting from type string to type int"

    // assertThrown can be used to assert that an exception is thrown
    assertThrown!ConvException("abc".to!int);

    // ifThrown can be used to provide a default value if an exception is thrown
    writeln("x".to!int().ifThrown(0)); // 0

    // handle is a more advanced version of ifThrown for ranges
    auto r = "12,1337z32,54".splitter(',').map!(a => to!int(a));
    auto h = r.handle!(ConvException, RangePrimitive.front, (e, r) => 0);
    assert(h.equal([12, 0, 54]));
    assertThrown!ConvException(h.retro.equal([54, 0, 12]));

    // basicExceptionCtors avoids the boilerplate when creating custom exceptions
    static class MeaCulpa : Exception
    {
        mixin basicExceptionCtors;
    }
    e = collectException((){throw new MeaCulpa("diagnostic message");}());
    writeln(e.msg); // "diagnostic message"
    writeln(e.file); // __FILE__
    writeln(e.line); // __LINE__ - 3

    // assumeWontThrow can be used to cast throwing code into `nothrow`
    void exceptionFreeCode() nothrow
    {
        // auto-decoding only throws if an invalid UTF char is given
        assumeWontThrow("abc".front);
    }

    // assumeUnique can be used to cast mutable instance to an `immutable` one
    // use with care
    char[] str = "  mutable".dup;
    str[0 .. 2] = "im";
    immutable res = assumeUnique(str);
    writeln(res); // "immutable"

auto `assertNotThrown`(T : Throwable = Exception, E)(lazy E `expression`, string `msg` = null, string `file` = __FILE__, size_t `line` = __LINE__);
  Asserts that the given expression does not throw the given type of Throwable. If a Throwable of the given type is thrown, it is caught and does not escape assertNotThrown. Rather, an AssertError is thrown. However, any other Throwables will escape.
  Parameters:

  T | The Throwable to test for.
  E `expression` | The expression to test.
  string `msg` | Optional message to output on test failure. If msg is empty, and the thrown exception has a non-empty msg field, the exception's msg field will be output on test failure.
  string `file` | The file where the error occurred. Defaults to __FILE__.
  size_t `line` | The line where the error occurred. Defaults to __LINE__.

  Throws:
  AssertError if the given Throwable is thrown.
  Returns:
  the result of `expression`.
  Examples:

      import core.exception : AssertError;

      import std.string;
      assertNotThrown!StringException(enforce!StringException(true, "Error!"));

      //Exception is the default.
      assertNotThrown(enforce!StringException(true, "Error!"));

      assert(collectExceptionMsg!AssertError(assertNotThrown!StringException(
                 enforce!StringException(false, "Error!"))) ==
             `assertNotThrown failed: StringException was thrown: Error!`);

void `assertThrown`(T : Throwable = Exception, E)(lazy E `expression`, string `msg` = null, string `file` = __FILE__, size_t `line` = __LINE__);
  Asserts that the given expression throws the given type of Throwable. The Throwable is caught and does not escape assertThrown. However, any other Throwables will escape, and if no Throwable of the given type is thrown, then an AssertError is thrown.
  Parameters:

  T | The Throwable to test for.
  E `expression` | The expression to test.
  string `msg` | Optional message to output on test failure.
  string `file` | The file where the error occurred. Defaults to __FILE__.
  size_t `line` | The line where the error occurred. Defaults to __LINE__.

  Throws:
  AssertError if the given Throwable is not thrown.
  Examples:

      import core.exception : AssertError;
      import std.string;

      assertThrown!StringException(enforce!StringException(false, "Error!"));

      //Exception is the default.
      assertThrown(enforce!StringException(false, "Error!"));

      assert(collectExceptionMsg!AssertError(assertThrown!StringException(
                 enforce!StringException(true, "Error!"))) ==
             `assertThrown failed: No StringException was thrown.`);

template `enforce`(E : Throwable = Exception) if (is(typeof(new E("", string.init, size_t.init)) : Throwable) || is(typeof(new E(string.init, size_t.init)) : Throwable))
T `enforce`(T, Dg, string file = __FILE__, size_t line = __LINE__)(T `value`, scope Dg `dg`)
if (isSomeFunction!Dg && is(typeof(`dg`())) && is(typeof(() { if (!`value`) { } } )));
T `enforce`(T)(T `value`, lazy Throwable `ex`);
  Enforces that the given value is true. If the given value is false, an exception is thrown. The
  - msg - error message as a string
  - `dg` - custom delegate that return a string and is only called if an exception occurred
  - `ex` - custom exception to be thrown. It is lazy and is only created if an exception occurred
  Parameters:

  T `value` | The value to test.
  E | Exception type to throw if the value evaluates to false.
  const(char)[] msg | The error message to put in the exception if it is thrown.
  Dg `dg` | The delegate to be called if the value evaluates to false.
  Throwable `ex` | The exception to throw if the value evaluates to false.
  string file | The source file of the caller.
  size_t line | The line number of the caller.

  Returns:
  `value`, if cast(bool) `value` is true. Otherwise, depending on the chosen overload, new Exception(msg), `dg`() or `ex` is thrown.

  Note `enforce` is used to throw exceptions and is therefore intended to aid in error handling. It is not intended for verifying the logic of your program. That is what assert is for. Also, do not use `enforce` inside of contracts (i.e. inside of in and out blocks and invariants), because contracts are compiled out when compiling with -release.

  If a delegate is passed, the safety and purity of this function are inferred from Dg's safety and purity.
  Examples:

      import core.stdc.stdlib : malloc, free;
      import std.conv : ConvException, to;

      // use enforce like assert
      int a = 3;
      enforce(a > 2, "a needs to be higher than 2.");

      // enforce can throw a custom exception
      enforce!ConvException(a > 2, "a needs to be higher than 2.");

      // enforce will return it's input
      enum size = 42;
      auto memory = enforce(malloc(size), "malloc failed")[0 .. size];
      scope(exit) free(memory.ptr);

  Examples:

      assertNotThrown(enforce(true, new Exception("this should not be thrown")));
      assertThrown(enforce(false, new Exception("this should be thrown")));

  Examples:

      writeln(enforce(123)); // 123

      try
      {
          enforce(false, "error");
          assert(false);
      }
      catch (Exception e)
      {
          writeln(e.msg); // "error"
          writeln(e.file); // __FILE__
          writeln(e.line); // __LINE__ - 7
      }

  Examples:
  Alias your own enforce function

      import std.conv : ConvException;
      alias convEnforce = enforce!ConvException;
      assertNotThrown(convEnforce(true));
      assertThrown!ConvException(convEnforce(false, "blah"));

  T `enforce`(T)(T `value`, lazy const(char)[] `msg` = null, string `file` = __FILE__, size_t `line` = __LINE__)
  if (is(typeof(() { if (!`value`) { } } )));
alias `errnoEnforce` = enforce!(ErrnoException).enforce(T)(T value, lazy const(char)[] msg = null, string file = __FILE__, size_t line = __LINE__) if (is(typeof(() { if (!value) { } } )));
  Enforces that the given value is true, throwing an ErrnoException if it is not.
  Parameters:

  T value | The value to test.
  const(char)[] msg | The message to include in the ErrnoException if it is thrown.

  Returns:
  value, if cast(bool) value is true. Otherwise, new ErrnoException(msg) is thrown. It is assumed that the last operation set errno to an error code corresponding with the failed condition.
  Examples:

      import core.stdc.stdio : fclose, fgets, fopen;
      import std.file : thisExePath;
      import std.string : toStringz;

      auto f = fopen(thisExePath.toStringz, "r").errnoEnforce;
      scope(exit) fclose(f);
      char[100] buf;
      auto line = fgets(buf.ptr, buf.length, f);
      enforce(line !is null); // expect a non-empty line

T `collectException`(T = Exception, E)(lazy E `expression`, ref E `result`);
  Catches and returns the exception thrown from the given expression. If no exception is thrown, then null is returned and `result` is set to the result of the expression.
  Note that while `collectException` can be used to collect any Throwable and not just Exceptions, it is generally ill-advised to catch anything that is neither an Exception nor a type derived from Exception. So, do not use `collectException` to collect non-Exceptions unless you're sure that that's what you really want to do.
  Parameters:

  T | The type of exception to catch.
  E `expression` | The expression which may throw an exception.
  E `result` | The result of the expression if no exception is thrown.

  Examples:

      int b;
      int foo() { throw new Exception("blah"); }
      assert(collectException(foo(), b));

      version (D_NoBoundsChecks) {}
      else
      {
          // check for out of bounds error
          int[] a = new int[3];
          import core.exception : RangeError;
          assert(collectException!RangeError(a[4], b));
      }

T `collectException`(T : Throwable = Exception, E)(lazy E `expression`);
  Catches and returns the exception thrown from the given expression. If no exception is thrown, then null is returned. E can be void.
  Note that while `collectException` can be used to collect any Throwable and not just Exceptions, it is generally ill-advised to catch anything that is neither an Exception nor a type derived from Exception. So, do not use `collectException` to collect non-Exceptions unless you're sure that that's what you really want to do.
  Parameters:

  T | The type of exception to catch.
  E `expression` | The expression which may throw an exception.

  Examples:

      int foo() { throw new Exception("blah"); }
      writeln(collectException(foo()).msg); // "blah"

string `collectExceptionMsg`(T = Exception, E)(lazy E `expression`);
  Catches the exception thrown from the given expression and returns the msg property of that exception. If no exception is thrown, then null is returned. E can be void.
  If an exception is thrown but it has an empty message, then emptyExceptionMsg is returned.
  Note that while `collectExceptionMsg` can be used to collect any Throwable and not just Exceptions, it is generally ill-advised to catch anything that is neither an Exception nor a type derived from Exception. So, do not use `collectExceptionMsg` to collect non-Exceptions unless you're sure that that's what you really want to do.
  Parameters:

  T | The type of exception to catch.
  E `expression` | The expression which may throw an exception.

  Examples:

      void throwFunc() { throw new Exception("My Message."); }
      writeln(collectExceptionMsg(throwFunc())); // "My Message."

      void nothrowFunc() {}
      assert(collectExceptionMsg(nothrowFunc()) is null);

      void throwEmptyFunc() { throw new Exception(""); }
      writeln(collectExceptionMsg(throwEmptyFunc())); // emptyExceptionMsg

enum string `emptyExceptionMsg`;
  Value that collectExceptionMsg returns when it catches an exception with an empty exception message.
pure nothrow immutable(T)[] `assumeUnique`(T)(T[] `array`);
pure nothrow immutable(T)[] `assumeUnique`(T)(ref T[] `array`);
pure nothrow immutable(T[U]) `assumeUnique`(T, U)(ref T[U] `array`);
  Casts a mutable array to an immutable array in an idiomatic manner. Technically, `assumeUnique` just inserts a cast, but its name documents assumptions on the part of the caller. `assumeUnique`(arr) should only be called when there are no more active mutable aliases to elements of arr. To strengthen this assumption, `assumeUnique`(arr) also clears arr before returning. Essentially assumeUnique(arr) indicates commitment from the caller that there is no more mutable access to any of arr's elements (transitively), and that all future accesses will be done through the immutable array returned by `assumeUnique`.
  Typically, `assumeUnique` is used to return arrays from functions that have allocated and built them.
  Parameters:

  T[] `array` | The array to cast to immutable.

  Returns:
  The immutable array.

  Example

      string letters()
      {
        char[] result = new char['z' - 'a' + 1];
        foreach (i, ref e; result)
        {
          e = cast(char)('a' + i);
        }
        return assumeUnique(result);
      }

  The use in the example above is correct because result was private to letters and is inaccessible in writing after the function returns. The following example shows an incorrect use of `assumeUnique`.

  Bad

      private char[] buffer;
      string letters(char first, char last)
      {
        if (first >= last) return null; // fine
        auto sneaky = buffer;
        sneaky.length = last - first + 1;
        foreach (i, ref e; sneaky)
        {
          e = cast(char)('a' + i);
        }
        return assumeUnique(sneaky); // BAD
      }

  The example above wreaks havoc on client code because it is modifying arrays that callers considered immutable. To obtain an immutable array from the writable array buffer, replace the last line with:

      return to!(string)(sneaky); // not that sneaky anymore

  The call will duplicate the array appropriately.
  Note that checking for uniqueness during compilation is possible in certain cases, especially when a function is marked as a pure function. The following example does not need to call assumeUnique because the compiler can infer the uniqueness of the array in the pure function:

      string letters() pure
      {
        char[] result = new char['z' - 'a' + 1];
        foreach (i, ref e; result)
        {
          e = cast(char)('a' + i);
        }
        return result;
      }

  For more on infering uniqueness see the unique and lent keywords in the ArchJava language.
  The downside of using `assumeUnique`'s convention-based usage is that at this time there is no formal checking of the correctness of the assumption; on the upside, the idiomatic use of `assumeUnique` is simple and rare enough to be tolerable.
  Examples:

      int[] arr = new int[1];
      auto arr1 = arr.assumeUnique;
      static assert(is(typeof(arr1) == immutable(int)[]));
      writeln(arr); // null
      writeln(arr1); // [0]

  Examples:

      int[string] arr = ["a":1];
      auto arr1 = arr.assumeUnique;
      static assert(is(typeof(arr1) == immutable(int[string])));
      writeln(arr); // null
      writeln(arr1.keys); // ["a"]

nothrow T `assumeWontThrow`(T)(lazy T `expr`, string `msg` = null, string `file` = __FILE__, size_t `line` = __LINE__);
  Wraps a possibly-throwing expression in a nothrow wrapper so that it can be called by a nothrow function.
  This wrapper function documents commitment on the part of the caller that the appropriate steps have been taken to avoid whatever conditions may trigger an exception during the evaluation of `expr`. If it turns out that the expression does throw at runtime, the wrapper will throw an AssertError.
  (Note that Throwable objects such as AssertError that do not subclass Exception may be thrown even from nothrow functions, since they are considered to be serious runtime problems that cannot be recovered from.)
  Parameters:

  T `expr` | The expression asserted not to throw.
  string `msg` | The message to include in the AssertError if the assumption turns out to be false.
  string `file` | The source file name of the caller.
  size_t `line` | The line number of the caller.

  Returns:
  The value of `expr`, if any.
  Examples:

      import std.math.algebraic : sqrt;

      // This function may throw.
      int squareRoot(int x)
      {
          if (x < 0)
              throw new Exception("Tried to take root of negative number");
          return cast(int) sqrt(cast(double) x);
      }

      // This function never throws.
      int computeLength(int x, int y) nothrow
      {
          // Since x*x + y*y is always positive, we can safely assume squareRoot
          // won't throw, and use it to implement this nothrow function. If it
          // does throw (e.g., if x*x + y*y overflows a 32-bit value), then the
          // program will terminate.
          return assumeWontThrow(squareRoot(x*x + y*y));
      }

      writeln(computeLength(3, 4)); // 5

pure nothrow @nogc @trusted bool `doesPointTo`(S, T, Tdummy = void)(auto const ref S `source`, const ref T `target`)
if (__traits(isRef, `source`) || isDynamicArray!S || isPointer!S || is(S == class));
pure nothrow @trusted bool `doesPointTo`(S, T)(auto const shared ref S `source`, const shared ref T `target`);
pure nothrow @trusted bool `mayPointTo`(S, T, Tdummy = void)(auto const ref S `source`, const ref T `target`)
if (__traits(isRef, `source`) || isDynamicArray!S || isPointer!S || is(S == class));
pure nothrow @trusted bool `mayPointTo`(S, T)(auto const shared ref S `source`, const shared ref T `target`);
  Checks whether a given source object contains pointers or references to a given target object.
  Parameters:

  S `source` | The source object
  T `target` | The target object

  Bugs:
  The function is explicitly annotated @nogc because inference could fail, see issue 17084.
  Returns:
  true if `source`'s representation embeds a pointer that points to `target`'s representation or somewhere inside it.
  If `source` is or contains a dynamic array, then, then these functions will check if there is overlap between the dynamic array and `target`'s representation.
  If `source` is a class, then it will be handled as a pointer.
  If `target` is a pointer, a dynamic array or a class, then these functions will only check if `source` points to `target`, not what `target` references.
  If `source` is or contains a union or void[n], then there may be either false positives or false negatives:
  `doesPointTo` will return true if it is absolutely certain `source` points to `target`. It may produce false negatives, but never false positives. This function should be prefered when trying to validate input data.
  `mayPointTo` will return false if it is absolutely certain `source` does not point to `target`. It may produce false positives, but never false negatives. This function should be prefered for defensively choosing a code path.

  Note Evaluating doesPointTo(x, x) checks whether x has internal pointers. This should only be done as an assertive test, as the language is free to assume objects don't have internal pointers (TDPL 7.1.3.5).

  Examples:
  Pointers

      int  i = 0;
      int* p = null;
      assert(!p.doesPointTo(i));
      p = &i;
      assert( p.doesPointTo(i));

  Examples:
  Structs and Unions

      struct S
      {
          int v;
          int* p;
      }
      int i;
      auto s = S(0, &i);

      // structs and unions "own" their members
      // pointsTo will answer true if one of the members pointsTo.
      assert(!s.doesPointTo(s.v)); //s.v is just v member of s, so not pointed.
      assert( s.p.doesPointTo(i)); //i is pointed by s.p.
      assert( s  .doesPointTo(i)); //which means i is pointed by s itself.

      // Unions will behave exactly the same. Points to will check each "member"
      // individually, even if they share the same memory

  Examples:
  Arrays (dynamic and static)

      int i;
       // trick the compiler when initializing slice
       // https://issues.dlang.org/show_bug.cgi?id=18637
      int* p = &i;
      int[]  slice = [0, 1, 2, 3, 4];
      int[5] arr   = [0, 1, 2, 3, 4];
      int*[]  slicep = [p];
      int*[1] arrp   = [&i];

      // A slice points to all of its members:
      assert( slice.doesPointTo(slice[3]));
      assert(!slice[0 .. 2].doesPointTo(slice[3])); // Object 3 is outside of the
                                                    // slice [0 .. 2]

      // Note that a slice will not take into account what its members point to.
      assert( slicep[0].doesPointTo(i));
      assert(!slicep   .doesPointTo(i));

      // static arrays are objects that own their members, just like structs:
      assert(!arr.doesPointTo(arr[0])); // arr[0] is just a member of arr, so not
                                        // pointed.
      assert( arrp[0].doesPointTo(i));  // i is pointed by arrp[0].
      assert( arrp   .doesPointTo(i));  // which means i is pointed by arrp
                                        // itself.

      // Notice the difference between static and dynamic arrays:
      assert(!arr  .doesPointTo(arr[0]));
      assert( arr[].doesPointTo(arr[0]));
      assert( arrp  .doesPointTo(i));
      assert(!arrp[].doesPointTo(i));

  Examples:
  Classes

      class C
      {
          this(int* p){this.p = p;}
          int* p;
      }
      int i;
      C a = new C(&i);
      C b = a;

      // Classes are a bit particular, as they are treated like simple pointers
      // to a class payload.
      assert( a.p.doesPointTo(i)); // a.p points to i.
      assert(!a  .doesPointTo(i)); // Yet a itself does not point i.

      //To check the class payload itself, iterate on its members:
      ()
      {
          import std.traits : Fields;

          foreach (index, _; Fields!C)
              if (doesPointTo(a.tupleof[index], i))
                  return;
          assert(0);
      }();

      // To check if a class points a specific payload, a direct memmory check
      // can be done:
      auto aLoc = cast(ubyte[__traits(classInstanceSize, C)]*) a;
      assert(b.doesPointTo(*aLoc)); // b points to where a is pointing

class `ErrnoException`: object.Exception;
  Thrown if errors that set errno occur.
  Examples:

      import core.stdc.errno : EAGAIN;
      auto ex = new ErrnoException("oh no", EAGAIN);
      writeln(ex.errno); // EAGAIN

  Examples:
  errno is used by default if no explicit error code is provided

      import core.stdc.errno : errno, EAGAIN;

      auto old = errno;
      scope(exit) errno = old;

      // fake that errno got set by the callee
      errno = EAGAIN;
      auto ex = new ErrnoException("oh no");
      writeln(ex.errno); // EAGAIN

  final pure nothrow @nogc @property scope @safe uint `errno`();
    Operating system error code.
  @safe this(string `msg`, string `file` = null, size_t `line` = 0);
    Constructor which takes an error message. The current global core.stdc.errno.errno value is used as error code.
  @safe this(string `msg`, int `errno`, string `file` = null, size_t `line` = 0);
    Constructor which takes an error message and error code.
CommonType!(T1, T2) `ifThrown`(E : Throwable = Exception, T1, T2)(lazy scope T1 `expression`, lazy scope T2 `errorHandler`);
CommonType!(T1, T2) `ifThrown`(E : Throwable, T1, T2)(lazy scope T1 `expression`, scope T2 delegate(E) `errorHandler`);
CommonType!(T1, T2) `ifThrown`(T1, T2)(lazy scope T1 `expression`, scope T2 delegate(Exception) `errorHandler`);
  ML-style functional exception handling. Runs the supplied expression and returns its result. If the expression throws a Throwable, runs the supplied error handler instead and return its result. The error handler's type must be the same as the expression's type.
  Parameters:

  E | The type of Throwables to catch. Defaults to Exception
  T1 | The type of the expression.
  T2 | The return type of the error handler.
  T1 `expression` | The expression to run and return its result.
  T2 `errorHandler` | The handler to run if the expression throwed.

  Returns:
  expression, if it does not throw. Otherwise, returns the result of errorHandler.
  Examples:
  Revert to a default value upon an error:

      import std.conv : to;
      writeln("x".to!int.ifThrown(0)); // 0

  Examples:
  Chain multiple calls to ifThrown, each capturing errors from the entire preceding expression.

      import std.conv : ConvException, to;
      string s = "true";
      assert(s.to!int.ifThrown(cast(int) s.to!double)
                     .ifThrown(cast(int) s.to!bool) == 1);

      s = "2.0";
      assert(s.to!int.ifThrown(cast(int) s.to!double)
                     .ifThrown(cast(int) s.to!bool) == 2);

      // Respond differently to different types of errors
      alias orFallback = (lazy a)  => a.ifThrown!ConvException("not a number")
                                       .ifThrown!Exception("number too small");

      writeln(orFallback(enforce("x".to!int < 1).to!string)); // "not a number"
      writeln(orFallback(enforce("2".to!int < 1).to!string)); // "number too small"

  Examples:
  The expression and the errorHandler must have a common type they can both be implicitly casted to, and that type will be the type of the compound expression.

      // null and new Object have a common type(Object).
      static assert(is(typeof(null.ifThrown(new Object())) == Object));
      static assert(is(typeof((new Object()).ifThrown(null)) == Object));

      // 1 and new Object do not have a common type.
      static assert(!__traits(compiles, 1.ifThrown(new Object())));
      static assert(!__traits(compiles, (new Object()).ifThrown(1)));

  Examples:
  Use a lambda to get the thrown object.

      import std.format : format;
      // "std.format.FormatException"
      writeln("%s".format.ifThrown!Exception(e => e.classinfo.name));

enum `RangePrimitive`: int;
  This enum is used to select the primitives of the range to handle by the handle range wrapper. The values of the enum can be OR'd to select multiple primitives to be handled.
  `RangePrimitive`.access is a shortcut for the access primitives; front, back and opIndex.
  `RangePrimitive`.pop is a shortcut for the mutating primitives; popFront and popBack.
  Examples:

      import std.algorithm.comparison : equal;
      import std.algorithm.iteration : map, splitter;
      import std.conv : to, ConvException;

      auto s = "12,1337z32,54,2,7,9,1z,6,8";

      // The next line composition will throw when iterated
      // as some elements of the input do not convert to integer
      auto r = s.splitter(',').map!(a => to!int(a));

      // Substitute 0 for cases of ConvException
      auto h = r.handle!(ConvException, RangePrimitive.front, (e, r) => 0);
      assert(h.equal([12, 0, 54, 2, 7, 9, 0, 6, 8]));

  Examples:

      import std.algorithm.comparison : equal;
      import std.range : retro;
      import std.utf : UTFException;

      auto str = "hello\xFFworld"; // 0xFF is an invalid UTF-8 code unit

      auto handled = str.handle!(UTFException, RangePrimitive.access,
              (e, r) => ' '); // Replace invalid code points with spaces

      assert(handled.equal("hello world")); // `front` is handled,
      assert(handled.retro.equal("dlrow olleh")); // as well as `back`

  `front`
  `back`
  `popFront`
  `popBack`
  `empty`
  `save`
  `length`
  `opDollar`
  `opIndex`
  `opSlice`
  `access`
  `pop`
auto `handle`(E : Throwable, RangePrimitive primitivesToHandle, alias handler, Range)(Range `input`)
if (isInputRange!Range);
  Handle exceptions thrown from range primitives.
  Use the RangePrimitive enum to specify which primitives to handle. Multiple range primitives can be handled at once by using the OR operator or the pseudo-primitives RangePrimitive.access and RangePrimitive.pop. All handled primitives must have return types or values compatible with the user-supplied handler.
  Parameters:

  E | The type of Throwable to handle.
  primitivesToHandle | Set of range primitives to handle.
  handler | The callable that is called when a handled primitive throws a Throwable of type E. The handler must accept arguments of the form E, ref IRange and its return value is used as the primitive's return value whenever E is thrown. For opIndex, the handler can optionally recieve a third argument; the index that caused the exception.
  Range `input` | The range to handle.

  Returns:
  A wrapper struct that preserves the range interface of `input`.

  Note Infinite ranges with slicing support must return an instance of std.range.Take when sliced with a specific lower and upper bound (see std.range.primitives.hasSlicing); `handle` deals with this by takeing 0 from the return value of the handler function and returning that when an exception is caught.

  Examples:

      import std.algorithm.comparison : equal;
      import std.algorithm.iteration : map, splitter;
      import std.conv : to, ConvException;

      auto s = "12,1337z32,54,2,7,9,1z,6,8";

      // The next line composition will throw when iterated
      // as some elements of the input do not convert to integer
      auto r = s.splitter(',').map!(a => to!int(a));

      // Substitute 0 for cases of ConvException
      auto h = r.handle!(ConvException, RangePrimitive.front, (e, r) => 0);
      assert(h.equal([12, 0, 54, 2, 7, 9, 0, 6, 8]));

  Examples:

      import std.algorithm.comparison : equal;
      import std.range : retro;
      import std.utf : UTFException;

      auto str = "hello\xFFworld"; // 0xFF is an invalid UTF-8 code unit

      auto handled = str.handle!(UTFException, RangePrimitive.access,
              (e, r) => ' '); // Replace invalid code points with spaces

      assert(handled.equal("hello world")); // `front` is handled,
      assert(handled.retro.equal("dlrow olleh")); // as well as `back`

template `basicExceptionCtors`()
  Convenience mixin for trivially sub-classing exceptions
  Even trivially sub-classing an exception involves writing boilerplate code for the constructor to: 1) correctly pass in the source file and line number the exception was thrown from; 2) be usable with enforce which expects exception constructors to take arguments in a fixed order. This mixin provides that boilerplate code.
  Note however that you need to mark the mixin line with at least a minimal (i.e. just ///) DDoc comment if you want the mixed-in constructors to be documented in the newly created Exception subclass.
  Current limitation: Due to bug #11500, currently the constructors specified in this mixin cannot be overloaded with any other custom constructors. Thus this mixin can currently only be used when no such custom constructors need to be explicitly specified.
  Examples:

      class MeaCulpa: Exception
      {
          ///
          mixin basicExceptionCtors;
      }

      try
          throw new MeaCulpa("test");
      catch (MeaCulpa e)
      {
          writeln(e.msg); // "test"
          writeln(e.file); // __FILE__
          writeln(e.line); // __LINE__ - 5
      }

  pure nothrow @nogc @safe this(string `msg`, string `file` = __FILE__, size_t `line` = __LINE__, Throwable `next` = null);
    Parameters:

    string `msg` | The message for the exception.
    string `file` | The file where the exception occurred.
    size_t `line` | The line number where the exception occurred.
    Throwable `next` | The previous exception in the chain of exceptions, if any.

  pure nothrow @nogc @safe this(string `msg`, Throwable `next`, string `file` = __FILE__, size_t `line` = __LINE__);
    Parameters:

    string `msg` | The message for the exception.
    Throwable `next` | The previous exception in the chain of exceptions.
    string `file` | The file where the exception occurred.
    size_t `line` | The line number where the exception occurred.

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