Conditional Compilation

Conditional compilation chooses which code is compiled, while normal variables still choose runtime data.

conditional_compilation.cpp

#include <iostream>
#include <string>

#define USE_SHORT_LABEL 1

int main() {
    int count = ;

#if USE_SHORT_LABEL
    std::string label = "qty";
#else
    std::string label = "quantity";
#endif

    std::cout << label << "=" << count << std::endl;
    std::cout << "double=" << count * 2 << std::endl;
    return 0;
}

#include <iostream>
#include <string>

#define USE_SHORT_LABEL 1

int main() {
    int count = ;

#if USE_SHORT_LABEL
    std::string label = "qty";
#else
    std::string label = "quantity";
#endif

    std::cout << label << "=" << count << std::endl;
    std::cout << "double=" << count * 2 << std::endl;
    return 0;
}

#include <iostream>
#include <string>

#define USE_SHORT_LABEL 1

int main() {
    int count = ;

#if USE_SHORT_LABEL
    std::string label = "qty";
#else
    std::string label = "quantity";
#endif

    std::cout << label << "=" << count << std::endl;
    std::cout << "double=" << count * 2 << std::endl;
    return 0;
}

conditional compilation `#if` and `#ifdef` decide which source text is kept before compilation.