Cannot add const to references?

This is a discussion on Cannot add const to references? within the C++ Programming forums, part of the General Programming Boards category; I'm having problems, and I'm wondering if this is normal or nor and if there's some workaround. Consider: Code: namespace ...

  1. #1
    C++まいる!Cをこわせ! Elysia's Avatar
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    Cannot add const to references?

    I'm having problems, and I'm wondering if this is normal or nor and if there's some workaround.
    Consider:

    Code:
    namespace mpl = Stuff::mpl;
    
    template<typename T>
    struct foo { typedef const T type; };
    
    int main()
    {
    	A a;
    	a.x = 0;
    	cout << mpl::type_name<foo<A&>::type>::name << endl;
    }
    mpl::type_name is a meta-template struct helper that helps in printing out names of the types since Microsoft's typeid.name() ignores const and reference identifiers on the types.
    Regardless of that, this does not work as expected. You would expect it to return (or I would) const A&, but it doesn't. It's still the same old A&.

    Visual Studio warns me:
    Warning 1 warning C4181: qualifier applied to reference type; ignored g:\w00t\visual studio 2008\projects\temp\temp2.cpp 35
    Which essentially means I cannot append const to a reference type.

    So is there a way to add const to a reference type?
    The idea is to do:

    void foo(typename mpl::ArgumentHelper<T>::const_type Arg)

    And const_type will be T if it's a native type and const T& if it's a class or struct.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  2. #2
    Registered User
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    Yes, what you're experiencing is "normal", or at least ... expected.

    I may not have fully understood your intent, but my guess is that you're looking for something like this;
    Code:
    template<typename T> class MyTraitsClass
    {
         typedef const T & const_type;
    };
    
    template<> class MyTraitsClass<short>
    {
        typedef short const_type;
    };
    
    // other specialisations for the other basic types.
    
    template<typename T>  class TypeStripper
    {
          typedef typename T actual_type;
    };
    
    // and a series of partial specialisations for reference types
    
    template<typename T &>  class TypeStripper
    {
          typedef typename T actual_type;
    };
    
    template<typename const T &>  class TypeStripper
    {
          typedef typename T actual_type;
    };
    
    template<typename T> class ArgumentHelper
    {
        typedef typename MyTraitsClass<TypeStripper<T>::actual_type>::const_type const_type; 
           // may need typename keyword more than once in here:  check that
    };

  3. #3
    C++まいる!Cをこわせ! Elysia's Avatar
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    Strip the reference, add const and re-add the reference!
    Works like a charm! Thanks!
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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