When can the standard library functions throw exceptions?

Question

I'm having trouble understanding which functions from the standard library can throw exceptions, and if so which and when.

Some functions are noexcept, in which case ok they don't, but if I look for example at std::string::operator[](): https://en.cppreference.com/w/cpp/string/basic_string/operator_at, the operator is not noexcept yet the page says nothing about exceptions (and same here so it should have nothing to do with std::basic_string being a template).

Can I assume that if no exception is listed on a page then it means that the function cannot throw (but was not marked as noexcept for some reason)? Or does it mean the function can throw implementation-defined exceptions? And when exceptions are listed, are they the only ones that can be thrown?

Edit:

• Someone remarked that I should refer to the actual standard and not cppreference for my question, and kindly provided me a relevant link.

• In my example the relevant page would be this one: https://eel.is/c++draft/string.access which explicitly states Throws: Nothing for operator[]()

• That being said, a few lines below is front(), which is not noexcept and doesn't have a Throws entry in its description, so my question still stands

Answer 1

Sometimes you have to read between the lines.

The string operator[] has a precondition that the index must be valid. cppreference says:

If pos > size(), the behavior is undefined.

Undefined behavior includes throwing exceptions. And, in fact, some implementations will throw an out-of-range exception in debug mode.

The math functions, like std::abs, originally come from the C library. Even though the C library doesn't throw exceptions, it also doesn't say noexcept for any of the functions, so to be compatible neither does C++.