CODEWITHSUNDEEP
c++pointersconstantsconstexpr
TheBlueCat
TheBlueCat

Reputation: 1195

constexpr initializing with pointers

I am trying to initialize a constexpr declaration with a pointer to int which is a const object. I also try to define an object with a object that is not a const type.

Code:

#include <iostream>

int main()
{
constexpr int *np = nullptr; // np is a constant to int that points to null;
int j = 0;
constexpr int i = 42; // type of i is const int
constexpr const int *p = &i; // p is a constant pointer to the const int i;
constexpr int *p1 = &j; // p1 is a constant pointer to the int j; 
}

g++ log:

constexpr.cc:8:27: error: ‘& i’ is not a constant expression
constexpr.cc:9:22: error: ‘& j’ is not a constant expression

I believe it is because the objects in main have no fixed addresses, thus g++ is throwing error messages back at me; how would I correct this? Without using literal types.

Upvotes: 15

Views: 12865

Answers (2)

Hui Lin
Hui Lin

Reputation: 1

#include <iostream>
// define i/j outside the function body, as mentioned in the book "c++ primer"
int j = 0;
constexpr int i = 42; // type of i is const int
int main()
{
   constexpr int *np = nullptr; // np is a constant to int that points to null;
   constexpr const int *p = &i; // p is a constant pointer to the const int i;
   constexpr int *p1 = &j; // p1 is a constant pointer to the int j; 
}

Upvotes: -1

Jesse Good
Jesse Good

Reputation: 52365

Make them static to fix their addresses:

int main()
{
  constexpr int *np = nullptr; // np is a constant to int that points to null;
  static int j = 0;
  static constexpr int i = 42; // type of i is const int
  constexpr const int *p = &i; // p is a constant pointer to the const int i;
  constexpr int *p1 = &j; // p1 is a constant pointer to the int j; 
}

This is known as an address constant expression [5.19p3]:

An address constant expression is a prvalue core constant expression of pointer type that evaluates to the address of an object with static storage duration, to the address of a function, or to a null pointer value, or a prvalue core constant expression of type std::nullptr_t.

Upvotes: 22

Related Questions