C++ - Compiler Directives

Introduction

Compiler directives are instructions that are not part of C++ / C standards, but tell the compiler how to compile the code. They starts with # (pronounced as “hash”), and are handled in the pre-processing phase and before the actual compilation.

• #include
• #pragmas pragmas - compiler-specific instructions
• #define macros
• attributes

Macros

Macros are string substitution for code. It happens during the preprocessing phase. It is generally not encouraged in c++ nowadays. However, there are still applications where it could be handy / necessary

Here is the general compilation process

Application 1 - Linkage Specifier

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#ifdef __cplusplus
    #define CV_IMPL extern "C"
#else
    #define CV_IMPL
#endif

CV_IMPL void cvFindExtrinsicCameraParams2(...){
    cvConvertPointsHomogeneous(objectPoints, matM);
    cvConvertPointsHomogeneous(imagePoints, _m);
}

This is effectively extern "C" void cvFindExtrinsicCameraParams2(...){} and void cvFindExtrinsicCameraParams2(...){}.

• extern "C" is a “linkage specifier”. In C++, function names are mangled so they will be unique in the object code. E.g., foo() could become _fooid1(). This is also the underlying mechanism for function overloading. However, C does not have function overloading. So, when linking C code in C++, we need to make sure names of function symbols are unique, and exclude name mangling.

Pragmas

GCC pragmas

• #pragma GCC / #pragma clang / #pragma warning: control warnings, optimizations
  • Diagnostic pragmas:

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      #pragma GCC diagnostic push
      #pragma GCC diagnostic ignored "-Wpedantic"
      #include "sophus/se3.hpp"
      #pragma GCC diagnostic pop  // recover diagnostic directive 
    

• #pragma once: ensuring a header is included once
• #pragma pack(n) - control data alignment.

Attributes / Annotations

Some compilers allow attributes to be added to functions / variables / types:

• __attribute__((...)) in GCC/Clang
• __declspec(...) in MSVC

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