getline: template argument deduction/substitution failed

Question

I'm getting this error when I try to read the output of a command line by line:

std::string exec(const char* cmd) {
  FILE* pipe = popen(cmd, "r");
  if (!pipe) return "";

  char buffer[128];
  std::string result = "";

  while (!feof(pipe)) {
    if (fgets(buffer, 128, pipe) != NULL) {
      result += buffer;
    }
  }

  pclose(pipe);

  return result;
}

string commandStr = "echo Hello World";
const char *command = commandStr.c_str();

std::string output = exec(command);

std::string line; 
while (std::getline(output, line)) {
  send(sockfd, line.c_str(), line.length(), 0); 
}

note: template argument deduction/substitution failed: note:
'std::string {aka std::basic_string}' is not derived from 'std::basic_istream<_CharT, _Traits>' exec(command), line <- Error

Answer 1

To solve the cause of the error message, I would guess it is from std::getline() . getline requires an std::istream as its first argument. If you want to read from output , you can use std::stringstream

std::stringstream ss(output);
while (std::getline(ss, line)) {
    /* ... */
}

Answer 2

std::getline(output, line)

You're trying to call getline with two strings, that's wrong, it takes an istream and a string.

To use getline you need to put the output into a stream:

std::istringstream outstr(output);
getline(outstr, line);