C++ 标准中 int 和 long 类型大小

1、标准规定的基本要求

C 和 C++ 标准没有明确规定 int、long、short 等整数类型在字节（byte）或位（bit）上的具体大小，而是定义了它们之间的相对大小关系和最小宽度。

1）整型大小的关系

#include <iostream>

int main() {
    std::cout << "sizeof(char)      = " << sizeof(char) << " byte(s)" << std::endl;
    std::cout << "sizeof(short)     = " << sizeof(short) << " byte(s)" << std::endl;
    std::cout << "sizeof(int)       = " << sizeof(int) << " byte(s)" << std::endl;
    std::cout << "sizeof(long)      = " << sizeof(long) << " byte(s)" << std::endl;
    std::cout << "sizeof(long long) = " << sizeof(long long) << " byte(s)" << std::endl;

    bool cond1 = sizeof(char) <= sizeof(short);
    bool cond2 = sizeof(short) <= sizeof(int);
    bool cond3 = sizeof(int) <= sizeof(long);
    bool cond4 = sizeof(long) <= sizeof(long long);

    if (cond1 && cond2 && cond3 && cond4) {
        std::cout << "The condition sizeof(char) <= sizeof(short) <= sizeof(int) <= sizeof(long) <= sizeof(long long) holds true." << std::endl;
    } else {
        std::cout << "The condition does NOT hold true on this system." << std::endl;
    }

    return 0;
}

注意：输出可能因平台和编译器不同而略有差异，但 C++ 标准只要求这些类型的大小顺序是递增的。

2）最小位宽（最低要求）

2、实际应用

跨平台时不要假设大小一致，不同平台下 long 可能是 32 位或 64 位。推荐使用 提供的固定宽度类型（如 int32_t, int64_t）来保证一致性。使用 sizeof 运算符查看类型在当前编译器下的实际字节数。

#include <iostream>
#include <cstdint>

int main() {
    std::cout << "sizeof(int): " << sizeof(int) << " bytes\n";
    std::cout << "sizeof(long): " << sizeof(long) << " bytes\n";
    std::cout << "sizeof(long long): " << sizeof(long long) << " bytes\n";

    // 使用固定宽度类型
    int32_t a = 123;
    int64_t b = 4567890123;

    std::cout << "int32_t: " << a << "\n";
    std::cout << "int64_t: " << b << "\n";

    return 0;
}