Preprocessor and Build Boundaries

Macro Constants

An object-like macro can name a compile-time text replacement, while a normal variable still changes at runtime.

macro constant A macro constant is replaced by the preprocessor before the compiler sees the program.

Macro Constants

subtotal
macro_constants.cpp
Replay: real traced execution (multi-file project)
#include <iostream>

#define TAX_PERCENT 8

#if TAX_PERCENT == 8
constexpr int taxPercent = 8;
#else
constexpr int taxPercent = 0;
#endif

int main() {
    int subtotal = 25;

    int tax = subtotal * taxPercent / 100;
    int total = subtotal + tax;

    std::cout << "subtotal=" << subtotal << std::endl;
    std::cout << "tax=" << tax << std::endl;
    std::cout << "total=" << total << std::endl;
    return 0;
}

#include <iostream>

#define TAX_PERCENT 8

#if TAX_PERCENT == 8
constexpr int taxPercent = 8;
#else
constexpr int taxPercent = 0;
#endif

int main() {
    int subtotal = 10;

    int tax = subtotal * taxPercent / 100;
    int total = subtotal + tax;

    std::cout << "subtotal=" << subtotal << std::endl;
    std::cout << "tax=" << tax << std::endl;
    std::cout << "total=" << total << std::endl;
    return 0;
}

#include <iostream>

#define TAX_PERCENT 8

#if TAX_PERCENT == 8
constexpr int taxPercent = 8;
#else
constexpr int taxPercent = 0;
#endif

int main() {
    int subtotal = 40;

    int tax = subtotal * taxPercent / 100;
    int total = subtotal + tax;

    std::cout << "subtotal=" << subtotal << std::endl;
    std::cout << "tax=" << tax << std::endl;
    std::cout << "total=" << total << std::endl;
    return 0;
}

  1. subtotal ← 25, tax ← 2, total ← 27

    11int main() {12    int subtotal→ 25 = 25; //@subtotal=10, 401314    int tax→ 2 = subtotal25 * taxPercent8 / 100;15    int total→ 27 = subtotal25 + tax2;1617    std::cout << "subtotal=" << subtotal25 << std::endl;18    std::cout << "tax=" << tax2 << std::endl;19    std::cout << "total=" << total27 << std::endl;20    return 0;21}
    outputsubtotal=25
tax=2
total=27

  1. subtotal ← 10, tax ← 0, total ← 10

    11int main() {12    int subtotal→ 10 = 10;1314    int tax→ 0 = subtotal10 * taxPercent8 / 100;15    int total→ 10 = subtotal10 + tax0;1617    std::cout << "subtotal=" << subtotal10 << std::endl;18    std::cout << "tax=" << tax0 << std::endl;19    std::cout << "total=" << total10 << std::endl;20    return 0;21}
    outputsubtotal=10
tax=0
total=10

  1. subtotal ← 40, tax ← 3, total ← 43

    11int main() {12    int subtotal→ 40 = 40;1314    int tax→ 3 = subtotal40 * taxPercent8 / 100;15    int total→ 43 = subtotal40 + tax3;1617    std::cout << "subtotal=" << subtotal40 << std::endl;18    std::cout << "tax=" << tax3 << std::endl;19    std::cout << "total=" << total43 << std::endl;20    return 0;21}
    outputsubtotal=40
tax=3
total=43