CODEWITHSUNDEEP
c++c++11decltype
Jim Garrison
Jim Garrison

Reputation: 4276

Using decltype to get an expression's type, without the const

Consider the following program:

int main ()
{
    const int e = 10;

    for (decltype(e) i{0}; i < e; ++i) {
        // do something
    }
}

This fails to compile with clang (as well as gcc):

decltype.cpp:5:35: error: read-only variable is not assignable
    for (decltype(e) i{0}; i < e; ++i) {
                                  ^ ~

Basically, the compiler is assuming that i must be const, since e is.

Is there a way I can use decltype to get the type of e, but removing the const specifier?

Upvotes: 58

Views: 18724

Answers (5)

M.M
M.M

Reputation: 141576

A solution not mentioned yet:

for (decltype(+e) i{0}; i < e; ++i)

Prvalues of primitive type have const stripped; so +e is a prvalue of type int and therefore decltype(+e) is int.

Upvotes: 10

klaus
klaus

Reputation: 826

You can also use std::decay:

#include<type_traits>
...
for (std::decay<decltype(e)>::type i{}; i < e; ++i) {
  // do something
}

Upvotes: 16

Khurshid Normuradov
Khurshid Normuradov

Reputation: 1604

I prefer range-for. Simulate it is very easy.

#include <iostream>

template< typename T >
struct range_t
{
    struct iter
    {
        T operator * ()const noexcept { return n;}
        iter& operator ++()noexcept{ ++n; return *this;}
        friend
        bool operator != (iter const& lhs, iter const& rhs)noexcept
        { return lhs.n != rhs.n;}

        T n;
    };

    iter begin()const noexcept {return {b};}
    iter end() const noexcept{ return {e};}
    T b, e;
};
template< typename T > range_t<T>  range(T b, T e){ return {b,e}; }

int main()
{
    const int e = 10;

    for( auto i : range(0,e) )
    {
        std::cout << i << ' ';
    }
    return 0;
}

Upvotes: 0

Casey
Casey

Reputation: 42554

I prefer auto i = decltype(e){0}; for this. It's a bit simpler than using type_traits, and I feel it more explicitly specifies the intent that you want a variable initialized to a 0 of e's type.

I've been using Herb's "AAA Style" a lot lately, so it could just be bias on my part.

Upvotes: 60

zch
zch

Reputation: 15278

Use std::remove_const:

#include<type_traits>
...
for (std::remove_const<decltype(e)>::type i{0}; i < e; ++i)

Upvotes: 55

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