Is this constexpr integer not a null pointer constant?
c++, c++11, constexpr
Solution
`[C++11: 5.19/3]:` A literal constant expression is a prvalue core constant expression of literal type, but not pointer type. An integral constant expression is a literal constant expression of integral or unscoped enumeration type. [..]
And:
`[C++11: 3.9/10]:` A type is a literal type if it is:
- a scalar type; or
- a reference type; or
- a class type (Clause 9) that has all of the following properties: [..]
- an array of literal type.
At this point, I can't find a reason for that code to be non-compliant, so I suspect a GCC bug.
However it may be a deliberate bug given that the passage you quoted out of 4.10 is proposed to be changed (active issue #903) so that this would in fact be non-compliant code.
The compilation succeeds if I `s/constexpr/const/` and compile with `-ansi` rather than `-std=c++11`.
The definition of integral constant expression explicitly allowed this case in C++03:
`[C++03: 5.19/1]:` [..] An integral constant-expression can involve only literals (2.13), enumerators, const variables or static data members of integral or enumeration types initialized with constant expressions (8.5), non-type template parameters of integral or enumeration types, and `sizeof` expressions. [..]
Problem
Consider the following C++11 program, and its result in GCC 4.7.2: ``` int main() { constexpr int i = 0; int* p = i; } // g++ -g -ggdb -Wall -Wextra -pedantic -std=c++11 t.cpp // t.cpp: In function 'int main()': // t.cpp:4:13: error: invalid conversion from 'int' to 'int*' [-fpermissive] // t.cpp:4:9: warning: unused variable 'p' [-Wunused-variable] ``` According to the standard: `[C++11: 4.10/1]:` A null pointer constant is an integral constant expression (5.19) prvalue of integer type that evaluates to zero [..] 5.19 is a mess and I've failed to parse it fully, but wouldn't we expect `i` to satisfy this criterion and act as a null pointer constant, consequently requiring no explicit conversion to `int*` for the initialisation of `p`? The compilation succeeds if I `s/constexpr/const/` and compile with `-ansi` rather than `-std=c++11`.