C++头文件解决问题场景小结

//-------------- math_tools.hpp --------------
#pragma once
double add(double, double);          // 声明
//-------------- math_tools.cpp --------------
double add(double a, double b) { return a + b; }
//-------------- main.cpp --------------------
#include "math_tools.hpp"
#include <iostream>
int main(){
    std::cout << add(3, 4);          // 5
}

//------------- circle.hpp ------------------
#pragma once
class Circle{                       // 公开接口
public:
    Circle(double r);
    double area() const;
private:
    double radius_;
};
//------------- circle.cpp ------------------
#include "circle.hpp"
#include <numbers>
Circle::Circle(double r): radius_(r) {}
double Circle::area() const{
    return std::numbers::pi * radius_ * radius_;
}
//------------- main.cpp --------------------
#include "circle.hpp"
#include <iostream>
int main(){
    Circle c(1.0);
    std::cout << c.area();          // 3.14159...
}

//-------------- utils.hpp ------------------
#pragma once
template<class T>
T max(T a, T b){ return (a > b) ? a : b; }

inline int sqr(int x){ return x * x; }
//-------------- main.cpp -------------------
#include "utils.hpp"
#include <iostream>
int main(){
    std::cout << max(3.5, 2.7) << '\n'; // 3.5
    std::cout << sqr(5) << '\n';        // 25
}

//-------------- config.hpp -----------------
#pragma once
#define VERSION        202506L
#define DEBUG_LOG(x)   do{ if constexpr(DEBUG) std::cerr << x << '\n'; }while(0)
constexpr bool DEBUG = true;
//-------------- main.cpp -------------------
#include "config.hpp"
#include <iostream>
int main(){
    DEBUG_LOG("debug mode on");
    std::cout << "ver " << VERSION;
}

//-------------- foo_fwd.hpp --------------  // 仅前向声明
#pragma once
class Foo;                                  // 不完整类型
void process(const Foo&);                   // 接受引用/指针即可
//-------------- foo.hpp -------------------  // 完整定义
#pragma once
#include "foo_fwd.hpp"
class Foo{ int value{}; };
void process(const Foo& f){ /*...*/ }
//-------------- main.cpp -------------------
#include "foo.hpp"
int main(){ Foo f; process(f); }