Question

I’m currently working on some logging code that supposed to – among other things – print information about the calling function. This should be relatively easy, standard C++ has a type_info class. This contains the name of the typeid’d class/function/etc. but it’s mangled. It’s not very useful. I.e. typeid(std::vector<int>).name() returns St6vectorIiSaIiEE.

Is there a way to produce something useful from this? Like std::vector<int> for the above example. If it only works for non-template classes, that’s fine too.

The solution should work for gcc, but it would be better if I could port it. It’s for logging so it’s not so important that it can’t be turned off, but it should be helpful for debugging.

Answer 1

Given the attention this question / answer receives, and the valuable feedback from GManNickG, I have cleaned up the code a little bit. Two versions are given: one with C++11 features and another one with only C++98 features.

In file type.hpp

#ifndef TYPE_HPP
#define TYPE_HPP

#include <string>
#include <typeinfo>

std::string demangle(const char* name);

template <class T>
std::string type(const T& t) {

    return demangle(typeid(t).name());
}

#endif

In file type.cpp (requires C++11)

#include "type.hpp"
#ifdef __GNUG__
#include <cstdlib>
#include <memory>
#include <cxxabi.h>

std::string demangle(const char* name) {

    int status = -4; // some arbitrary value to eliminate the compiler warning

    // enable c++11 by passing the flag -std=c++11 to g++
    std::unique_ptr<char, void(*)(void*)> res {
        abi::__cxa_demangle(name, NULL, NULL, &status),
        std::free
    };

    return (status==0) ? res.get() : name ;
}

#else

// does nothing if not g++
std::string demangle(const char* name) {
    return name;
}

#endif

Usage:

#include <iostream>
#include "type.hpp"

struct Base { virtual ~Base() {} };

struct Derived : public Base { };

int main() {

    Base* ptr_base = new Derived(); // Please use smart pointers in YOUR code!

    std::cout << "Type of ptr_base: " << type(ptr_base) << std::endl;

    std::cout << "Type of pointee: " << type(*ptr_base) << std::endl;

    delete ptr_base;
}

It prints:

Type of ptr_base: Base*
Type of pointee: Derived

Tested with g++ 4.7.2, g++ 4.9.0 20140302 (experimental), clang++ 3.4 (trunk 184647), clang 3.5 (trunk 202594) on Linux 64 bit and g++ 4.7.2 (Mingw32, Win32 XP SP2).

If you cannot use C++11 features, here is how it can be done in C++98, the file type.cpp is now:

#include "type.hpp"
#ifdef __GNUG__
#include <cstdlib>
#include <memory>
#include <cxxabi.h>

struct handle {
    char* p;
    handle(char* ptr) : p(ptr) { }
    ~handle() { std::free(p); }
};

std::string demangle(const char* name) {

    int status = -4; // some arbitrary value to eliminate the compiler warning

    handle result( abi::__cxa_demangle(name, NULL, NULL, &status) );

    return (status==0) ? result.p : name ;
}

#else

// does nothing if not g++
std::string demangle(const char* name) {
    return name;
}

#endif

---

---

(Update from Sep 8, 2013)

The accepted answer (as of Sep 7, 2013), when the call to abi::__cxa_demangle() is successful, returns a pointer to a local, stack allocated array… ouch!
Also note that if you provide a buffer, abi::__cxa_demangle() assumes it to be allocated on the heap. Allocating the buffer on the stack is a bug (from the gnu doc): “If output_buffer is not long enough, it is expanded using realloc.” Calling realloc() on a pointer to the stack… ouch! (See also Igor Skochinsky‘s kind comment.)

You can easily verify both of these bugs: just reduce the buffer size in the accepted answer (as of Sep 7, 2013) from 1024 to something smaller, for example 16, and give it something with a name not longer than 15 (so realloc() is not called). Still, depending on your system and the compiler optimizations, the output will be: garbage / nothing / program crash.
To verify the second bug: set the buffer size to 1 and call it with something whose name is longer than 1 character. When you run it, the program almost assuredly crashes as it attempts to call realloc() with a pointer to the stack.

---

(The old answer from Dec 27, 2010)

Important changes made to KeithB’s code: the buffer has to be either allocated by malloc or specified as NULL. Do NOT allocate it on the stack.

It’s wise to check that status as well.

I failed to find HAVE_CXA_DEMANGLE. I check __GNUG__ although that does not guarantee that the code will even compile. Anyone has a better idea?

#include <cxxabi.h>

const string demangle(const char* name) {

    int status = -4;

    char* res = abi::__cxa_demangle(name, NULL, NULL, &status);

    const char* const demangled_name = (status==0)?res:name;

    string ret_val(demangled_name);

    free(res);

    return ret_val;
}