C++问题，为什么回车两次才输出结果呢？？

int main()
{
string line;
// read line at time until end-of-file
while (getline(cin, line))
cout << line << endl;
return 0;
}

因为getline有三个参数，第三个参数是字符串的结束符，也就是当getline遇到这个结束符时，就不在接受输入了，getline默认情况下以回车('\n')作为结束符，第一次按回车表示字符串结束，第二次按回车才开始输出。
你可以通过多读一次char的方法放掉这个回车符。这个在C++编程思想里有很详细的解释，你可以找来看看的。

FIX: getline Template Function Reads Extra Character

RESOLUTION

Modify the getline member function, which can be found in the following system header file "string", as follows:

else if (_Tr::eq((_E)_C, _D))
{_Chg = true;

// _I.rdbuf()->snextc(); // Remove this line and add the line below.
_I.rdbuf()->sbumpc();
break; }

STATUS

Microsoft has confirmed that this is a bug in the Microsoft products that are listed at the beginning of this article.

This problem was corrected in Microsoft Visual C++ .NET.

MORE INFORMATION

The following sample program demonstrates the bug: //test.cpp

#include
#include
int main () {
std::string s,s2;
std::getline(std::cin,s);
std::getline(std::cin,s2);
std::cout << s <<'\t'<< s2 << std::endl;
return 0;
}

//Actual Results:

Hello
World

Hello World

//Expected Results:

Hello
World
Hello World

--------------------------------------------------------------------------------

APPLIES TO
• The Standard C++ Library, when used with:
Microsoft Visual C++ 6.0 Enterprise Edition
Microsoft Visual C++ 6.0 Professional Edition
Microsoft Visual C++ 6.0 Standard Edition

cin>>c; 修改为： cin.read(&c, 1); 函数>>是读串送给c，可以理解为是将输入的串送给c的一个命令； read是读1（第二个参数）个字节送给c，此时是个命令，同时也是一个字节，如果不输入任意字符直接输入的话，那么read将赋给了c，从而循环。