How to control #define directive in different projects ...? - c++

Question

My question is quite straight forward. I just intended to know that is the #define directive in C++ controllable over the different project files? Elaborately, I have a header file and a cpp file for one project. The codes of the files are as follows:
MyHeader.h
#ifndef __MY_HEADER_H__
#include <cstring>
using namespace std;
#ifdef _HEADER_EXPORT_
#define HEADER_API __declspec(dllexport)
#else
#define HEADER_API __declspec(dllimport)
#endif
#ifdef __cplusplus
extern "C" {
#endif
class HEADER_API MyHeader
{
public:
MyHeader();
~MyHeader();
#ifdef _HEADER_DISPLAY_
void __cdecl ParseHeader();
#elif defined (_HEADER_RETURN_)
string __cdecl ParseHeader();
#endif
};
#ifdef __cplusplus
}
#endif
#define __MY_HEADER_H__
#endif
MyHeader.cpp
#ifndef __MY_HEADER_H__
#include "MyHeader.h"
#endif
MyHeader::MyHeader() { }
MyHeader::~MyHeader() { }
#ifdef __cplusplus
extern "C" {
#endif
#ifdef _HEADER_DISPLAY_
HEADER_API void __cdecl MyHeader::ParseHeader()
{
fputs(string("Displaying...").c_str(), stdout);
}
#elif defined (_HEADER_RETURN_)
HEADER_API string __cdecl MyHeader::ParseHeader()
{
string retVal("Returning...");
return retVal;
}
#endif
#ifdef __cplusplus
}
#endif
In another project HeaderImpl.cpp file has been implemented with the following code.
HeaderImpl.cpp
#include "stdafx.h"
#define _HEADER_DISPLAY_ // To display the message
// #define _HEADER_RETURN_ // To return the message as string
#include "MyHeader.h"
int main(int argc, char* argv[])
{
MyHeader header;
MyHeader.ParseHeader(); // To display the message or to return the string
return 0;
}
Now, I wanted to know that how can I use the #define directive in my HeaderImpl.cpp file to control the ParseHeader method for MyHeader.cpp file? As it has been noted that MyHeader.h file doing exactly what I need for; i.e. controlling the ParseHeader method upon declaring the #define directive, accordingly.

Answer 1

You can't. Each C++ source file is compiled independently, and settings in one cannot affect another. You'll have to do this on project level.
One way to do that would be to set up different project (and solution) configurations for different values of this macro. Instead of just the usual Debug and Release, you could add Debug-Display, Debug-Return etc. You can then define the macros in the project settings for each configuration. This will make sure you link the correctly built version of your library.
As a side note, you're using illegal names in your code. A name which contains double underscores, or starts with an underscore followed by an uppercase letter, is reserved for the compiler & standard library. User code is not allowed to use such names for its own purposes.

Answer 2

You usually can provide #defines for all of your compilation units on the command line of the compiler. IIRC for Visual studio that would be something like /D_HEADER_DISPLAY_ or /D_HEADER_RETURN_
Your project must be using something like this already for the _HEADER_EXPORT_ define.

Answer 3

There is no way for a preprocessor definition in one translation unit to remotely affect a different translation unit.
Most, if not all, compilers accept them as parameters for the compilation, though (and the flag is usually -D, or /D for VC++).
In Visual Studio, you can set project-wide preprocessor definitions in the project settings, under
Configuration Properties -> C/C++ -> Preprocessor -> Preprocessor Definitions