constexpr initializing with pointers

Question

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.

Well, yeah. Variables only have addresses at runtime when they are assigned one. — Qaz
– Qaz, Commented Nov 21, 2012 at 18:24
@chris Yes. How would I define a function to have its objects to have a fixed address? Would I just declare the function as constexpr? — TheBlueCat
– TheBlueCat, Commented Nov 21, 2012 at 18:27
@chris Can you explain your point more? The whole point of constexpr is that is defined at compiletime with mainly literal types (they're expections). My point is objects within functions have temporary addresses assigned to them (for obvious reasons). Here g++ is throwing me the error messages due to those temp memory addresses. How could I declare a function that would not do this (ie, allow all objects to be fixed during control of flow). — TheBlueCat
– TheBlueCat, Commented Nov 21, 2012 at 18:32
I highly doubt you can associate addresses or pointers to variables with constexpr at all. — Qaz
– Qaz, Commented Nov 21, 2012 at 18:35
Unfortunately this example works fine in VS 2015. Poor Microsoft. — Lukáš Bednařík
– Lukáš Bednařík, Commented Nov 13, 2015 at 8:12

Jesse Good · Accepted Answer · 2012-11-21 21:50:36Z

22

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.

answered Nov 21, 2012 at 21:50

Jesse Good

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4 Comments

anton_rh Over a year ago

Isn't the address of a static variable defined at link-time, not at compile-time?

Jesse Good Over a year ago

@anton_rh: Yes, the address is determined at link-time. That does not conflict with my answer above though.

sh1 Over a year ago

@JesseGood I think it's a good point to elaborate on, though. That a constexpr can be a value resolved after compile time.

Goswin von Brederlow Over a year ago

If you look at the implementation then you see what actually happens is that the compiler will generate a constant + a linker reference how to fix up the constant relative to the link address.

Hui Lin · Accepted Answer · 2023-12-03 15:54:01Z

-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; 
}

answered Dec 3, 2023 at 15:54

Hui Lin

1

1 Comment

Community Over a year ago

As it’s currently written, your answer is unclear. Please edit to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center.

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