Introduction - D Programming Language
Description: D Programming Language
Keywords: D programming language

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# Introduction

D is a general-purpose systems programming language with a C-like syntax that compiles to native code. It is statically typed and supports both automatic (garbage collected) and manual memory management. D programs are structured as modules that can be compiled separately and linked with external libraries to create native libraries or executables.

This document is the reference manual for the D Programming Language. For more information and other documents, see The D Language Website.

## Phases of Compilation

The process of compiling is divided into multiple phases. Each phase is independent of subsequent phases. For example, the scanner is not affected by the semantic analyzer. This separation of passes makes language tools like syntax directed editors relatively easy to create. It is also possible to compress D source by storing it in ‘tokenized’ form.

1. source character set
   The source file is checked to see which encoding it is using, and the appropriate scanner is loaded. 7-bit ASCII and UTF encodings are accepted.
2. script line
   If the first line starts with "#!", then that line is ignored.
3. lexical analysis
   The source file is divided up into a sequence of tokens. Special tokens are replaced with other tokens. SpecialTokenSequences are processed and removed.
4. syntax analysis
   The sequence of tokens is parsed to form syntax trees.
5. semantic analysis
   The syntax trees are traversed to declare variables, load symbol tables, assign types, and in general determine the meaning of the program.
6. optimization
   Optimization is an optional pass that tries to rewrite the program in a semantically equivalent, but faster executing, version.
7. code generation
   Instructions are selected from the target architecture to implement the semantics of the program. The typical result will be an object file, suitable for input to a linker.

## Memory Model

A byte is the fundamental unit of storage. Each byte has 8 bits, and is stored at a unique address. A memory location is a sequence of one or more bytes of the exact size required to hold a scalar type. Multiple threads can access separate memory locations without interference.

Memory locations come in three groups:

1. Thread local memory locations are accessible from only one thread at a time.
2. Immutable memory locations cannot be written to during their lifetime. Immutable memory locations can be read from by multiple threads without synchronization.
3. Shared memory locations are accessible from multiple threads.

Undefined Behavior: Allowing multiple threads to access a thread local memory location results in undefined behavior.
Undefined Behavior: Writing to an immutable memory location during its lifetime results in undefined behavior.
Undefined Behavior: Unless synchronized, writing to a shared memory location in one thread while other threads read or write to it results in undefined behavior.

Execution of a single thread on thread local and immutable memory locations is sequentially consistent. This means the result of the operations is the same as if they were executed in the same order that the operations appear in the program.

A memory location can be transferred from thread local to immutable or shared if there is only one reference to the location.

A memory location can be transferred from shared to immutable or thread local if there is only one reference to the location.

A memory location can be temporarily transferred from shared to local if synchronization is used to prevent any other threads from accessing the memory location during the operation.

## Object Model

An object is created when one of the following is executed:

- a definition
- a NewExpression
- a temporary is created
- changing which field of a union is active

An object spans a sequence of memory locations which may or may not be contiguous. It exists during construction, its lifetime, and destruction. Each object has a type, which is determined either statically or by runtime type information. The memory locations may include any combination of thread local, immutable, or shared memory locations.

Objects can be composed into a composed object. The objects that make up the composed object are subobjects. An object that is not the subobject of another object is a complete object. The lifetime of a subobject is always within the lifetime of its complete object.

An object's address is the address of the first byte of the first memory location for that object. Object addresses are distinct from each other unless one of the objects is nested within the other.

Lexical
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