2.6

宏定义：

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#include <iostream>

#define PRINT_JOE

int main()
{
#ifdef PRINT_JOE
    std::cout << "Joe\n"; // will be compiled since PRINT_JOE is defined
#endif

#ifdef PRINT_BOB
    std::cout << "Bob\n"; // will be excluded since PRINT_BOB is not defined
#endif

    return 0;
}
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Joe

**#if 1 **和 **#if 0 **

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#include <iostream>

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

#if 0 // Don't compile anything starting here
    std::cout << "Bob\n";
    std::cout << "Steve\n";
#endif // until this point

    return 0;
}
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Joe
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#include <iostream>

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

#if 1 // always true, so the following code will be compiled
    std::cout << "Bob\n";
    /* Some
     * multi-line
     * comment here
     */
    std::cout << "Steve\n";
#endif

    return 0;
}
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Joe
Bob
Steve

为了防止文件头中的函数被多次包含，被重写而产生错误，我们可以用：

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#ifndef SOME_UNIQUE_NAME_HERE
#define SOME_UNIQUE_NAME_HERE

比如你有一个add.h

你就使用：

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#ifndef ADD_H
#define ADD_H

但是现在一般都是使用

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#pragma once