why is my compiler complaining when i try to clear the cin buffer

Question

i am writing a program where i am trying to implement the following code:

int main(){

string inputcmd;


while (getline(cin, inputcmd)){
    cout << "TYPE A COMMAND" << endl;   
    cin >> inputcmd;

    cin.ignore (std::numeric_limits<std::streamsize>::max(), '\n'); 

    if (inputcmd == "make"){
        cout << "MAKING NEW PROJECT" << endl;
        get_project(cin);
    }

    else if (inputcmd == "retrieve"){
        cout << "RETRIEVING YOUR PROJECT" << endl;
    }
}

return 0;
}

i am trying to use the cin.ignore property to clear the buffer of the newline character currently residing in the buffer at that given time, however when i try and compile it is giving me a bunch of gibberish compiler error? why is this how can i fix this?

When you post a question about build errors, always include the complete and unedited error log. No matter if it seems to be gibberish to you, or if it's a lot, it helps us to understand what's happening. — Some programmer dude, Nov 06 '13 at 08:48
By the way, you *do* include ``, which is needed for [`std::numeric_limits`](http://en.cppreference.com/w/cpp/types/numeric_limits)? (Another tip about posting code, please make a [SSCCE](http://sscce.org/) containing the problem, and post that complete.) — Some programmer dude, Nov 06 '13 at 08:49
Apart from the error message itself, you are missing the includes and using directives you seem to have used. Please provide an [SSCCE](http://sscce.org) — Arne Mertz, Nov 06 '13 at 08:49
i included limits and now it compiles. however now it for some reason just gets stuck in the first main while loop, the program runs and asks to TYPE A COMMAND but thats all it asks for, sometimes you have to press newline a couple of times before it ask for it again. how can i remedy this? — notamathwiz, Nov 06 '13 at 08:54

score 1 · Answer 1 · answered Nov 06 '13 at 08:55

1

Assuming you included

#include <string>
#include <iostream>
using namespace std;

Then I'm not getting any error.

answered Nov 06 '13 at 08:55

CroCo

5,531
9
56
88

Constantin · Answer 2 · 2013-11-06T09:14:08.163

You're using a strange combination of getline and cin... If you're using getline, you don't have to call cin.ignore at all. Don't mix both like you did or you will get confusing results.

This example propably runs like you want:

#include <string>
#include <iostream>

using namespace std;

int main(){
  string inputcmd;
  bool running = true;
  while (running){
    cout << "TYPE A COMMAND" << endl;
    getline(cin, inputcmd);
    if (inputcmd.substr(0, inputcmd.find(" ")) == "make"){
      if(inputcmd.find(" ")!=string::npos){
        inputcmd = inputcmd.substr(inputcmd.find(" ")+1);
        cout << "MAKING NEW PROJECT: " << inputcmd << endl;
        //get_project(cin);
      }else{
        cout << "make: not enough arguments" << endl;
      }
    }else if (inputcmd == "retrieve"){
      cout << "RETRIEVING YOUR PROJECT" << endl;
    }else if(inputcmd == ""){
      running = false;
    }
  }
  return 0;
}

You might want to put a condition around that `getline` call. What if the user presses the EOF key-combination or there is some error? — Some programmer dude, Nov 06 '13 at 08:59

Some programmer dude · Answer 3 · 2013-11-06T09:09:23.167

You need to press an extra newline because you read the input twice. First with getline and a second time with cin >> ....

If you can have arguments to the commands, I recommend you remove the cin >> ... part, together with the cin.ignore() call, and use only getline and std::istringstream:

std::cout << "Enter command: ";
while (std::getline(std::cin, line))
{
    std::istringstream iss(line);

    // Get the command
    std::string command;
    iss >> command;

    if (command == "make")
    {
        ...
    }
    ...

    std::cout << "Enter command: ";
}

This way, you can easily get the space-separated arguments to the command as well.

And yes, you have code to print the prompt twice, but that is a smaller and negligible problem in my opinion.

Or if you want to be even more general, use e.g. a std::vector to store the command and arguments, and do something like

std::cout << "Enter command: ";
while (std::getline(cin, line))
{
    std::istringstream iss(line);

    std::vector<std::string> args;

    // Get the command and all arguments, and put them into the `args` vector
    std::copy(std::istream_iterator<std::string>(iss),
              std::istream_iterator<std::string>(),
              std::back_inserter(args));

    if (args[0] == "make")
    {
        ...
    }
    ...

    std::cout << "Enter command: ";
}

See e.g. these references:

why is my compiler complaining when i try to clear the cin buffer

3 Answers3