C/C++ :: How To Choose Which Picture To Load Using CImg Library

I've just started using the CImg library and when I try to load a picture with this code:

CImg<unsigned char> image("example.bmp");

It works perfectly. But when I try to type in the image file name using the console it gives me errors. For example, if I try this:

std::string input_file;
std::cin >> input_file;
CImg<unsigned char> image(input_file);

It doesn't work. The way I see it, I should be able type "example.bmp" into input_file and use it but it doesn't work, and gives me the error:

"error: no matching function for call to 'cimg_library::CImg<unsigned char>::CImg(std::string&)"

I have included iostream and string.h.