Check if is const on C++03

c++, c++03

Solution

An example implementation for C++11's `is_const` is given on cppreference, and it looks like this:

template<class T> struct is_const          : false_type {};
template<class T> struct is_const<const T> : true_type {};

If you put this definition in your C++03 code, you can use `is_const` there as well, if you add definitions for `false_type` and `true_type` (thanks to mfonantini for pointing out the missing `true_type` and `false_type`). If you define them as follows, you'll get very close to the definition used in C++11:

struct true_type {
  static const bool value = true;
  typedef bool value_type;
  typedef true_type type;
  operator value_type() const { return value; }
};

struct false_type {
  static const bool value = false;
  typedef bool value_type;
  typedef false_type type;
  operator value_type() const { return value; }
};

The only difference is that the static `value` is a mere `const`, not a `constexpr`, but note that it is a constant expression nevertheless and can be used as template argument. So for all practical purposes, the definition above should work in C++03.

Regarding the last part of your question: There is actually no problem casting a non-const type to const. (Illegal situations can, however, arise with pointers to pointers or references to pointers, e.g. `T**` cannot be cast to `const T**`.)

Problem

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