C++ 为什么我的 HelloWorld 函数没有在这个范围内声明？
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Why is my HelloWorld function not declared in this scope?
提问by MatthewSot
#include <iostream>
using namespace std;
int main()
{
HelloWorld();
return 0;
}
void HelloWorld()
{
cout << "Hello, World" << endl;
}
I am getting the following compilation error with g++:
我在使用 g++ 时遇到以下编译错误：
l1.cpp: In function 'int main()':
l1.cpp:5:15: error: 'HelloWorld' was not declared in this scope
回答by Mysticial
You need to either declare or define the function before you can use it. Otherwise, it doesn't know that HelloWorld()exists as a function.
您需要先声明或定义该函数，然后才能使用它。否则，它不知道HelloWorld()作为函数存在。
Add this before your main function:
在你的主函数之前添加这个：
void HelloWorld();
Alternatively, you can move the definition of HelloWorld()before your main():
或者，您可以HelloWorld()在您的之前移动 的定义main()：
#include <iostream>
using namespace std;
void HelloWorld()
{
cout << "Hello, World" << endl;
}
int main()
{
HelloWorld();
return 0;
}
回答by Nasreddine
You must declare the function before you can use it:
您必须先声明该函数，然后才能使用它：
#include <iostream>
using namespace std;
void HelloWorld();
int main()
{
HelloWorld();
return 0;
}
void HelloWorld()
{
cout << "Hello, World" << endl;
}
or you can move the definition of HelloWorld()before main()
或者你可以移动HelloWorld()之前的定义main()
回答by matthias
You need to forward declare HelloWorld()so mainknows what it is. Like so:
您需要转发声明，HelloWorld()以便main知道它是什么。像这样：
#include <iostream>
using namespace std;
void HelloWorld();
int main()
{
HelloWorld();
return 0;
}
void HelloWorld()
{
cout << "Hello, World" << endl;
}
回答by Morten Kristensen
You need to have either a prototype of the function before the invocation or the whole function before it.
您需要在调用之前拥有函数的原型，或者在调用之前拥有整个函数。
So the first is:
所以第一个是：
void HelloWorld();
int main() {
HelloWorld();
return 0;
}
void HelloWorld() {
cout << "Hello, World" << endl;
}
And the second way is:
第二种方式是：
void HelloWorld() {
cout << "Hello, World" << endl;
}
int main() {
HelloWorld();
return 0;
}
回答by Gene Bushuyev
There is one more possibility for some reason nobody mentioned, namely using extern declaration:
由于某种原因没有人提到，还有另一种可能性，即使用 extern 声明：
#include <iostream>
using namespace std;
int main()
{
extern void HelloWorld();
HelloWorld();
return 0;
}
void HelloWorld()
{
cout << "Hello, World" << endl;
}
It's preferable when you don't want to introduce name of the function into namespace scope.
当您不想将函数的名称引入命名空间范围时，这是可取的。
回答by Walter
All these answers are correct, but strangely enough, this would have worked:
所有这些答案都是正确的，但奇怪的是，这会奏效：
struct Hello {
static int main() { World(); return 0; } /* note: World() not declared yet */
static void World() { std::cout<<"Hello World"; }
};
int main() { return Hello::main(); }
回答by Tusk
You have to put the function before the main function.
你必须把函数放在主函数之前。
回答by gprathour
in C++ you need to define or at least declare the functions before calling them.
在 C++ 中，您需要在调用函数之前定义或至少声明函数。
What you are trying to do till now is something like this :
到目前为止，您正在尝试做的是这样的：
int main()
{
cout << b;
int b = 10;
}
So you can also trying like this :
所以你也可以这样尝试：
#include <iostream>
using namespace std;
void HelloWorld();
int main()
{
HelloWorld();
return 0;
}
void HelloWorld()
{
cout << "Hello, World" << endl;
}
It is a good practice in C++ to define all other functions before the main function.
在 C++ 中，在 main 函数之前定义所有其他函数是一个很好的做法。
回答by moshbear
Rearrange HelloWorld()so that it appears before main():
重新排列HelloWorld()使其出现在 之前main()：
#include <iostream>
using namespace std;
void HelloWorld()
{
cout << "Hello, World" << endl;
}
int main()
{
HelloWorld();
return 0;
}
