C++ Question about .append() vs .push_back() for Strings and Characters (Beginner Question)

Question

I have a question regarding which functions work for appending strings.
In this particular code, it asks: Given string givenInput on one line and character extension on a second line, append extension to givenInput.
Ex: If the input is:
Hello !
then the output is:
Hello!

The code I have so far

int main() {
    string givenInput;
    char extension;

    getline(cin, givenInput);
    cin >> extension;
    
    // this is the line of code I am allowed to edit and have come up with:
    givenInput.push_back(extension);
    
    cout << givenInput << endl; 

Why is it that givenInput.push_back(extension); works but then givenInput.append(extension); does not?
I thought that append() works with types such as strings, characters, integers, etc..., but when I use the append function, it gives an error...
I actually originally had givenInput = givenInput.append(extension); , but there was an error, which then led me to do just givenInput.append(extension); (which also had an error), which then finally left me to try givenInput.push_back(extension); which ended up working.

In this other prompt, it asks: Assign secretID with firstName, a space, and lastName.
Ex: If firstName is John and lastName is Doe, then output is: John Doe

I have:

int main() {
   string secretID;
   string firstName;
   string lastName;

   cin >> firstName;
   cin >> lastName;
   
   // this is the line of code I can edit
   secretID = firstName.append(" " + lastName);

   cout << secretID << endl;

Why in this case would the append function work, but it wouldn't work for the other example?
Is it simply due to the fact that lastName is a string, while extension is a char?
Also, why is it that something like secretID = firstName.append(); works for this second example while something like givenInput = givenInput.append(extension) does not work for the first example?

I know this is A LOT of text, but if someone could please explain the rules behind this, that would be greatly appreciated, as it would clear up my confusion!!

***Note: I do understand that something like givenInput = givenInput + extension; as well as
secretID = firstName + " " + lastName; would work for both examples, but I guess I am asking simply about the rules that apply for the append() and push_back() functions, regardless of if they aren't the most optimal to use for these cases.

Answer 1

In the class std::string (or class template std::basic_string) there is no overloaded member function append that has only one parameter of the type char (or charT). For characters there is the following overloaded member function append

basic_string& append(size_type n, charT c);

So you could write for example

givenInput.append(1, extension);

The set of overloaded member functions append is similar to the constructors of the class std::string: in general member functions append have similar lists of parameters as constructors. Consider for example the following constructor

basic_string(size_type n, charT c, const Allocator& a = Allocator());

As you can see the above function append has the same parameters as the first two parameters of the constructor.

Pay attention to that instead of this statement

givenInput.push_back(extension);

you could just write

givenInput += extension;

Answer 2

Rather than trying functions out through trial and error, you could look at what member functions std::string provides:

• See reference for std::string::append
• See reference for std::string::push_back

Notice that append doesn't have any overload that takes just one character. If you really wanted to use it, you could theoretically call:

givenInput.append(1, extension);

... but that is rather pointless, because you could write:

givenInput.push_back(extension);
// or
givenInput += extension;

In the second example, you have used append correctly, because it does have an overload (2) that accepts a std::string.

When in doubt, just use operator+= because it works with both characters and strings:

secretID += ' ';
secretID += lastName;