if else和#if #else #endif之间的区别 [英] The differences between if else and #if #else #endif

问题描述

在 if / else 和 #if / #else /#之间，我感到困惑endif 构造.

1. 它们之间有什么区别?
2. 在每种情况下我都应该使用它们?

推荐答案

我对if/else和#if/#else/#endif感到困惑.看起来它们具有相同的逻辑功能.

I am confused about the if/else and #if/#else/#endif. It seems that they have the same logic functionality.

请问它们之间有什么区别?

Can I ask what's the differences between them?

#if ， #else 和 #endif 属于预处理.它们未执行，但是文本替换的说明.您可以将它们视为通常在文本编辑器中发现的一种自动搜索与替换"功能.

#if, #else and #endif belong to preprocessing. They are not executed but are instructions for textual replacement. You can think of them as a kind of automatic "search & replace" feature you'd usually find in a text editor.

if 和 else 是运行时结构.您可以认为它们是在程序运行时执行的.

if and else are run-time constructs. You can think of them as being executed while the program runs.

假设您有此程序:

#include <iostream>

#define VALUE 1

int main()
{
#if VALUE == 1
    std::cout << "one\n";
#else
    std::cout << "not one\n";
#endif
}

当您告诉编译器编译该程序时，预处理器将在实际编译真实" C ++代码之前进行文本替换.好像程序是:

When you tell your compiler to compile this program, the preprocessor will make a textual replacement before the "real" C++ code is actually compiled. It will be as if the program was:

#include <iostream>

int main()
{
    std::cout << "one\n";
}

现在，从技术上讲，您可以在此处使用 if :

Now, technically you could use an if here:

#include <iostream>

#define VALUE 1

int main()
{
    if (VALUE == 1)
    {
        std::cout << "one\n";
    }
    else
    {
        std::cout << "not one\n";
    }
}

但是在C ++中，您不要对常量使用 #define .相反，您会有类似的东西:

But in C++ you don't use #define for constants. You'd instead have something like:

#include <iostream>

int const value = 1;

int main()
{
    if (value == 1)
    {
        std::cout << "one\n";
    }
    else
    {
        std::cout << "not one\n";
    }
}

也许只有在程序执行时才知道该值，例如通过用户输入.那么您显然不能使用 #if ，它仅在程序运行之前起作用.您必须使用 if :

Perhaps the value is only known while the program executes, e.g. via user input. Then you obviously cannot use #if, which only works before the program runs. You must use if:

#include <iostream>

int main()
{
    int value;
    std::cin >> value; // gross simplification here

    if (value == 1)
    {
        std::cout << "one\n";
    }
    else
    {
        std::cout << "not one\n";
    }
}

What kind of specific situations for me to choose each of them?

对于初学者来说，一个好的指导方针是:仅将 #if (或实际上: #ifndef )用于

A good guideline for a beginner would be: Use #if (or actually: #ifndef) only for include guards. Consider further uses of #if when you encounter problems that can only be solved by the preprocessor.

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