Why does typeid.name() return weird characters usi

Question

How come when I run this main.cpp:

#include <iostream>
#include <typeinfo>

using namespace std;

struct Blah {};

int main() {
  cout << typeid(Blah).name() << endl;
  return 0;
}

By compiling it with GCC version 4.4.4:

g++ main.cpp

I get this:

4Blah

On Visual C++ 2008, I would get:

struct Blah

Is there a way to make it just print Blah or struct Blah?

Answer 1

typeid().name() is implementation dependent. It may even return empty string for every type. That would not be very useful implementation, but it would be valid.

Answer 2

Is there a way to make it just print

Blah or struct Blah?

No. The result of std::typeinfo::name() is unspecified. It might even return the same string for all types (or, indeed, empty strings for all types) and the implementation would still be standard-conforming. You must not rely on its result. Really, the only thing I found it useful for was debugging.

Tell us what what you need it for. Often traits is what you use instead.

Answer 3

The string returned is implementation defined.

What gcc is doing is returning the mangled name.
You can convert the mangled name into plain text with c++filt

> a.out | c++filt

Answer 4

in 4Blah, 4 is the number of letters in your class name. For example if your class name is myEmptyClass then it would print 12myEmptyClass.

Answer 5

The return of name is implementation defined : an implementation is not even required to return different strings for different types.

What you get from g++ is a decorated name, that you can "demangle" using the c++filt command or __cxa_demangle.

Answer 6

As others have said, the result here is implementation-defined, meaning that the implementation (i.e., the compiler toolchain) is free to define it how it wants, so long as it documents that somewhere.

From the C++ standard, section 18.5.1/1 [lib.type.info]:

The class type_info describes type information generated by the implementation. Objects of this class effectively store a pointer to a name for the type, and an encoded value suitable for comparing two types for equality or collating order. The names, encoding rule, and collating sequence for types are all unspecified and may differ between programs.