Moving code outside of main function, getting error "implicit instantiation of undefined template"

Question

I'm looking at implementing slurp, a function that reads in a file and returns a string:

When this code in a main() function, it works fine:

int main(){
  std::ifstream ifs("stuff.txt");
  std::string content((std::istreambuf_iterator<char>(ifs)),
                      (std::istreambuf_iterator<char>()));
  std::cout << content;
  return 0;
}

However, when I move the file out to a function in a different file:

std::string slurp(std::string filename){
  std::ifstream ifs(filename);
  std::string content((std::istreambuf_iterator<char>(ifs)),
                      (std::istreambuf_iterator<char>()));
  return content;
}

I get an error like this when compiling:

[ 33%] Building CXX object CMakeFiles/image_calibration.dir/source/cpp/kanya.cpp.o
/Users/chris/Development/chit/image-bench/source/cpp/kanya.cpp:15:19: error: implicit instantiation of undefined template 'std::__1::basic_ifstream<char,
std::__1::char_traits<char> >'
std::ifstream ifs(filename);
          ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/iosfwd:131:32: note: template is declared here
class _LIBCPP_TEMPLATE_VIS basic_ifstream;

What is happening?

Answer 1

Likely you're just missing:

#include <fstream>