Question

Note that in general, double is different from long double.

strtod converts string to double, but which function should be use to converting string to long double?

Answer 1

In C++03, use boost::lexical_cast, or:

std::stringstream ss(the_string);
long double ld;
if (ss >> ld) {
    // it worked
}

In C99, use strtold.

In C89, use sscanf with %Lg.

In C++11 use stold.

There may be subtle differences as to exactly which formats each one accepts, so check the details first…