类/函数顺序在C ++中重要吗？

问题描述

我开始学习C ++。在IDE代码块中，它会编译：

I'm beginning to learn C++. In the IDE codeblocks, this compiles:

#include <iostream>
using namespace std;

struct A {};

struct B {
    A a;
}

void hi() {
    cout << "hi" << endl;
}

int main() {
    hi();
    return 0;
}

但这不是：

struct B {
    A a;
}

struct A {};

int main() {
    hi();
    return 0;
}

void hi() {
    cout << "hi" << endl;
}

它给了我错误：

error: 'A' does not name a type
error: 'hi' was not declared in this scope

在C ++中类/函数的顺序应该重要吗？我以为不是。请澄清问题。

Should class/function order matter in C++? I thought it doesn't. Please clarify the issue.

推荐答案

是的，您必须至少声明类/函数，然后才能进行使用/调用它，即使之后才真正定义。

Yes, you must at least declare the class/function before you use/call it, even if the actual definition does not come until afterwards.

这就是为什么您经常在头文件中声明类/函数，然后在<$ c $中声明c> #include 放在cpp文件顶部。然后，您可以按任何顺序使用类/函数，因为它们已经被有效地声明了。

That is why you often declare the classes/functions in header files, then #include them at the top of your cpp file. Then you can use the classes/functions in any order, since they have already been effectively declared.

请注意，您可以这样做。 （）

Note in your case you could have done this. (working example)

void hi();    // This function is now declared

struct A; // This type is now declared

struct B {
    A* a; // we can now have a pointer to it
};

int main() {
    hi();
    return 0;
}

void hi() {    // Even though the definition is afterwards
    cout << "hi" << endl;
}

struct A {}; // now A has a definition

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