代码在ideone中编译，但不在gcc中编译

Question

I wrote the following code:

#include <iostream>
using namespace std;

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

I ran it in ideone , and it compiled successfully. I have the same code in file test1.cpp on my computer, I ran g++ test1.cpp and I got the following error:

./test1.cpp: In function ‘int main()’:
./test1.cpp:7:2: error: a function-definition is not allowed here before ‘return’

Why dose this happen? is this a bug? I'm using linux mint, gcc version 4.7.

Answer 1

This is commonly known as C++'s most vexing parse. When you do something like

int f();

the compiler reads this as a function prototype, declaring a function f that returns an int . If you're using C++11, you should instead do

int f{}; // f initialized to 0

if you're not using C++11, make sure to initialize the variable right away.

Answer 2

You are missing a semi-colon here:

 int v()
        ^

should be:

 int v() ;

which is a function declaration, not clear that was what was intended though. If you want to initialize v then the following would work:

 int v(0) ;

or in C++11 :

 int v{0} ; 

Answer 3

你忘了分号之后

int v();

Answer 4

Ideone is using gcc 4.8.1 for your code (as you can see in your own link) while you are using 4.7

There are several difference regarding C++ 11 implementation, and apparently it is affected by the line that looks like a function delcaration.