Set string to file contents c++

I was wanting to know if there was a simple way to set a std::string equal to the contents of a file in c++. So far, I was thinking something like this(although I haven't tested it, so I don't know if it will work),

#include <fstream>
#include <string>

int main(int argc, char *argv[]){

    fstream in("file.txt");
    string str;

    str = in;

    return 0;
}

Is this a way to accomplish this? If not, is there a simple way to do so? Thanks!