Weird behaviour with std::getline and std::vector<std::string>

I have the following code:

std::vector<std::string> final_output;
std::string input;

int tries = 0;
std::cin >> tries;

int counter = 0;
while(counter < tries) {

    std::getline(std::cin, input); 
    final_output.push_back(input);
    ++counter;

}

Given the input:

3
Here Goes
Here Goes 2

The output is:

<blank line>
Here Goes
Here Goes 2

Weirdly, it seems to enter a blank line as input for the first time it runs.

However, if I have the code as:

int tries = 3;         开发者_开发问答// explicitly specifying the number of tries
int counter = 0;
while(counter < tries) {}

It works as expected. Why is the std::cin >> tries causing the code to fail?

I have tested it with VC++ 2010 and g++ 4.4.3

When you enter the number for tries, you hit the return key. After you read tries, the carriage return from hitting the return key is still sitting in the input buffer. That carriage return will normally be translated to a new-line character. Your next call to getline reads everything in the input buffer up to the next new-line. Since the first character is a new-line, it reads that as a line of zero length (i.e., zero characters before the new-line).

The newline of the first entry is still in the input buffer.

You can call std::cin.ignore(); just after reading tries from cin.

This way the newline gets discarded.

I found a good link that explains plenty of things regarding the use of I/O:

• http://www.augustcouncil.com/~tgibson/tutorial/iotips.html

You have nothing to absorb the '\n' from the first line in your standalone std::cin >> tries.