Mryntu

In Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 (Section 10.4.5 Address Constant Expressions), he uses a code example where the address of a local variable is set to a constexpr variable. I thought this looked odd, so I tried running the example with g++ version 7.3.0 and was unable to get the same results. Here is his code example verbatim (although slightly abridged):

extern char glob;

void f(char loc) 
 constexpr const char* p0 = &glob; // OK: &glob's is a constant
 constexpr const char* p2 = &loc; // OK: &loc is constant in its scope

When I run this, I get:

error: ‘(const char*)(& loc)’ is not a constant expression

Is something happening with g++ that I'm not aware of, or is there something more to Bjarne's example?

Bjarne Stroustrup's book "The C++ Programming Language (4th Edition)" on p. 267 refers to the following code:

constexpr const char* p1="asdf";

This is OK because "asdf" is stored in a fixed memory location.

void f(char loc) 
 constexpr const char* p0 = &glob; // OK: &glob's is a constant
 constexpr const char* p2 = &loc; // OK: &loc is constant in its scope

However, loc is not in a fixed memory location. it's on the stack and will have varying locations depending on when it is called.

It appears that the example from section 10.4.5 provided in my hard-copy of the "The C++ Programming Language (4th Edition)" is incorrect. And so I've concluded that the address of a local variable is not a constexpr.

The example appears to have been updated in some pdf versions as seen here:

Just to add to other answers that have pointed out the mistake, C++ standard only allows constexpr pointers to objects of static-storage duration, one past the end of such, or nullptr. See [expr.const/8] specifically #8.2;

It's worth noting that:

• 
  string-literals have static-storage duration:


• Based on constraints in declaring extern variables, they'll inherently have static-storage duration or thread local-storage duration.

Hence this is valid:

#include <string>

extern char glob;
std::string boom = "Haha";

void f(char loc) 
 constexpr const char* p1 = &glob;
 constexpr std::string* p2 = nullptr;
 constexpr std::string* p3 = &boom;