Using an element of a constexpr array vs a const array to instantiate a template

Question

While answering a question, I ran into an issue that I couldn't explain.

It seems there is large enough difference between

constexpr size_t IntArray[2] = {1, 2};

and

const size_t IntArray[2] = {1, 2};

that the elements of the first can be used to instantiate a template but not the elements of the second.

Sample program that demonstrates the difference.

#include <cstddef>

template <size_t N> struct Foo {};

// Works fine
void test1()
{
   constexpr size_t IntArray[2] = {1, 2};
   const size_t i = IntArray[0];
   Foo<i> f;
   (void)f; // Remove unused f warning.
}

// Does not work
void test2()
{
   const size_t IntArray[2] = {1, 2};
   const size_t i = IntArray[0];
   Foo<i> f;
   (void)f; // Remove unused f warning.
}

int main()
{
   return 0;
}

I get the following compiler error using g++ 4.9.2.

g++ -std=c++11 -Wall    socc.cc   -o socc

socc.cc: In function ‘void test2()’:
socc.cc:17:8: error: the value of ‘i’ is not usable in a constant expression
    Foo<i> f;
        ^
socc.cc:16:17: note: ‘i’ was not initialized with a constant expression
    const size_t i = IntArray[0];
                 ^
socc.cc:17:9: error: the value of ‘i’ is not usable in a constant expression
    Foo<i> f;
         ^
socc.cc:16:17: note: ‘i’ was not initialized with a constant expression
    const size_t i = IntArray[0];
                 ^
socc.cc:17:9: note: in template argument for type ‘long unsigned int’
    Foo<i> f;
         ^
socc.cc:17:12: error: invalid type in declaration before ‘;’ token
    Foo<i> f;

My question is why does the constexpr array work but not the const array?

Why do you think constexpr was introduced? I bet the const array would work if it had static storage duration, mind you. Can you present the standard wording that you think should make it valid as it is? — Lightness Races in Orbit
– Lightness Races in Orbit, Commented May 29, 2015 at 16:30
@LightnessRacesinOrbit, Use of static didn't make any difference. I have some understanding of why constexpr was introduced but there are gaps. This question illustrates some of that gap. — R Sahu
– R Sahu, Commented May 29, 2015 at 16:36
Then I refer you again to the standard; specifically, [expr.const] and friends. Happy reading! — Lightness Races in Orbit
– Lightness Races in Orbit, Commented May 29, 2015 at 16:37
What would you expect with const size_t IntArray[2] = {1, bar()}; for constexpr size_t a = intArray[0 /* or 1 */];, it would be tricky and requires to remember for each index if it is a compile time value or not. — Jarod42
– Jarod42, Commented May 29, 2015 at 17:28
@Jarod42, thanks for that. I think that explains it. const does not guarantee the value can be determined at compile time while constexpr does. — R Sahu
– R Sahu, Commented May 29, 2015 at 17:38

Jarod42 · Accepted Answer · 2015-05-30 12:00:45Z

2

constexpr ensures that value is compile time value whereas const forbids only to modify the value.

Whereas it is easily to mark single const variable if it is a compile time value or not, for C-array, it would require to remember that information for each index.

Even if we can do

const int two = 2;
constexpr int two_bis = two;

the following is not permit

const size_t IntArray[2] = {1, bar()}; // with non constexpr bar()

constexpr size_t a = intArray[0]; // we might know that is compile time value
constexpr size_t b = intArray[1]; // but this one is clearly not

answered May 30, 2015 at 12:00

Jarod42

227k15 gold badges214 silver badges350 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Marco A. Over a year ago

precisely constexpr specifies that "it is possible to evaluate the value of the function or variable at compile time"

Collectives™ on Stack Overflow

Using an element of a constexpr array vs a const array to instantiate a template

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related