/D (Preprocessor Definitions)

Article
08/03/2021

Defines a preprocessing symbol for a source file.

Syntax

/D[ ]name[= | # [{ string | number }] ]
/D[ ]"name[= | # [{ string | number }] ]"

Remarks

You can use this symbol together with #if or #ifdef to compile source code conditionally. The symbol definition remains in effect until it's redefined in the code, or is undefined in the code by an #undef directive.

/D has the same effect as a #define directive at the beginning of a source code file. The difference is that /D strips quotation marks on the command line, and a #define directive keeps them. You can have whitespace between the /D and the symbol. There can't be whitespace between the symbol and the equals sign, or between the equals sign and any value assigned.

By default, the value associated with a symbol is 1. For example, /D name is equivalent to /D name=1. In the example at the end of this article, the definition of TEST is shown to print 1.

Compiling by using /D name= causes the symbol name to have no associated value. Although the symbol can still be used to conditionally compile code, it otherwise evaluates to nothing. In the example, if you compile by using /DTEST=, an error occurs. This behavior resembles the use of #define with or without a value.

The /D option doesn't support function-like macro definitions. To insert definitions that can't be defined on the command line, consider the /FI (Name forced include file) compiler option.

You can use /D multiple times on the command line to define more symbols. If the same symbol is defined more than once, the last definition is used.

This command defines the symbol DEBUG in TEST.c:

CL /DDEBUG TEST.C

This command removes all occurrences of the keyword __far in TEST.c:

CL /D __far= TEST.C

The CL environment variable can't be set to a string that contains the equal sign. To use /D together with the CL environment variable, you must specify the number sign (#) instead of the equal sign:

SET CL=/DTEST#0

When you define a preprocessing symbol at the command prompt, consider both compiler parsing rules and shell parsing rules. For example, to define a percent-sign preprocessing symbol (%) in your program, specify two percent-sign characters (%%) at the command prompt. If you specify only one, a parsing error is emitted.

CL /DTEST=%% TEST.C

To set this compiler option in the Visual Studio development environment

Open the project Property Pages dialog box. For more information, see Set C++ compiler and build properties in Visual Studio.
Select the Configuration Properties > C/C++ > Preprocessor property page.
Open the drop-down menu of the Preprocessor Definitions property and choose Edit.
In the Preprocessor Definitions dialog box, add, modify, or delete one or more definitions, one per line. Choose OK to save your changes.

You don't need to include the '/D' option prefix on the definitions you specify here. In the property page, definitions are separated by semicolons (;).

To set this compiler option programmatically

See PreprocessorDefinitions.

Example

// cpp_D_compiler_option.cpp
// compile with: cl /EHsc /DTEST cpp_D_compiler_option.cpp
#include <stdio.h>

int main( )
{
#ifdef TEST
    printf_s("TEST defined %d\n", TEST);
#else
    printf_s("TEST not defined\n");
#endif
}

TEST defined 1