questions about name mangling in C++

Mangling is simply how the compiler keeps the linker happy.

In C, you can't have two functions with the same name, no matter what. So that's what the linker was written to assume: unique names. (You can have static functions in different compilation units, because their names aren't of interest to the linker.)

In C++, you can have two functions with the same name as long as they have different parameter types. So C++ combines the function name with the types in some way. That way the linker sees them as having different names.

The exact manner of mangling is not significant to the programmer, only the compiler, and in fact every compiler does it differently. All that matters is that every function with the same base name is somehow made unique for the linker.

You can see now that adding namespaces and templates to the mix keeps extending the principle.

C does not do name mangling, though it does pre-pend an underscore to function names, so the printf(3) is actually _printf in the libc object.

In C++ the story is different. The history of it is that originally Stroustrup created "C with classes" or cfront, a compiler that would translate early C++ to C. Then rest of the tools - C compiler and linker would we used to produce object code. This implied that C++ names had to be translated to C names somehow. This is exactly what name mangling does. It provides a unique name for each class member and global/namespace function and variable, so namespace and class names (for resolution) and argument types (for overloading) are somehow included in the final linker names.

This is very easy to see with tools like nm(1) - compile your C++ source and look at the generated symbols. The following is on OSX with GCC:

namespace zoom
{
    void boom( const std::string& s )
    {
        throw std::runtime_error( s );
    }
}

~$ nm a.out | grep boom
0000000100001873 T __ZN4zoom4boomERKSs

In both C and C++ local (automatic) variables produce no symbols, but live in registers or on stack.

Edit:

Local variables do not have names in resulting object file for mere reason that linker does not need to know about them. So no name, no mangling. Everything else (that linker has to look at) is name-mangled in C++.