C++ error, compiler won't recognize string::push_back [closed] - c++

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The function it has an issue with:
string encode (string message, string key) {
string code = "whatever";
string forst;
int num;
string::size_type begin = 0;
message = lower_and_strip(message);
for (char val : message) {
num = return_encoded_char(key, begin, val);
forst = to_string(num);
code.push_back(forst); //*******************************
}
return code;
}
The starred line is what it points to. The return_encoded_char function returns an integer.
The specific error is
proj05.cpp:68:23: error: no matching function for call to 'std::basic_string<char>::push_back(std::string&)' and points to the line I starred.
I initially just declared code without initializing it, but changing that didn't fix it. All the similar questions I could find had some other element to blame; I feel like this should be relatively straightforward, though obviously it's not since it isn't working.
I have #include <stream> and using std::to_string etc. I'm using -std=c++11 to compile it.
Help.
P.S. Using Geany on Linux.

Answer 1

Your code variable is a std::string. The std::string class doesn't have a push_back() method that takes another std::string as input. You should try with the += operator instead, which accepts a character or a string:
string encode (string message, string key) {
string code = "whatever";
string forst;
int num;
string::size_type begin = 0;
message = lower_and_strip(message);
for (char val : message) {
num = return_encoded_char(key, begin, val);
forst = to_string(num);
code += forst; //*******************************
}
return code;
}