For people new to template resolution and template type deduction in C++, they may have written this code and get confused why it doesn’t compile:

#include <string>

int main()
    std::string s;
    s += 2;     // compiles correctly
    s = s + 2;  // compiler error
    return 0;

And here’s my answer:

TL;DR operator+=() is a class member function in class string, while operator+() is a template function.

The standard class template<typename CharT> basic_string<CharT> has an overloaded member function basic_string& operator+=(CharT), and string is just basic_string<char>.

As values that fits in a shorter type can be interpreted as that type, in the expression s += 2, the 2 is not treated as int, but char instead. It has exactly the same effect as s += '\x02'. A char with ASCII code 2 (STX) is appended, instead of the character '2' (with ASCII value 50, or 0x32).

However, string does not have an overloaded member function like string operator+(int), so s + 2 is not a valid expression, thus throws an error during compilation. (More below)

You can use operator+ function in string in these ways:

s = s + char(2);  // or (char)2
s = s + std::string(2);
s = s + std::to_string(2);  // C++11 and above only

For people concerned about why 2 isn’t automatically cast to char with operator+,

template <typename CharT>
  operator+(const basic_string<CharT>& lhs, CharT rhs);

The above is the prototype[note] for the plus operator in s + 2, and because it’s a template function, it is requiring an implementation of both operator+<char> and operator+<int>, which is conflicting. For details, see Why isn’t automatic downcasting applied to template functions?

Meanwhile, the prototype of operator+= is:

template <typename CharT>
class basic_string{
      operator+=(CharT _c);

You see, no template here (it’s a class member function), so the compiler deduces that type CharT is char from class implementation, and int(2) is automatically cast into char(2).

Note: Unnecessary code is stripped when copying from C++ standard include source. That includes typename 2 and 3 (Traits and Allocator) for template class “basic_string”, and unnecessary underscores, in order to improve readability.

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