constexpr const vs constexpr variables?

c++ c++11 constants constexpr
Thomas Moulard picture Thomas Moulard · Mar 4, 2015 · Viewed 18.8k times · Source

It seems obvious that constexpr implies const and thus it is common to see:

constexpr int foo = 42; // no const here

However if you write:

constexpr char *const str = "foo";

Then GCC will spawn "warning: deprecated conversion from string constant to ‘char*’" if -Wwrite-string flag is passed.

Writing:

constexpr const char *const str = "foo";

solves the issue.

So are constexpr const and constexpr really the same?

Answer

T.C. picture T.C. · Mar 4, 2015

The issue is that in a variable declaration, constexpr always applies the const-ness to the object declared; const on the other hand can apply to a different type, depending on the placement.

Thus

constexpr const int i = 3;
constexpr int i = 3;

are equivalent;

constexpr char* p = nullptr;
constexpr char* const p = nullptr;

are equivalent; both make p a const pointer to char.

constexpr const char* p = nullptr;
constexpr const char* const p = nullptr;

are equivalent. constexpr makes p a const pointer. The const in const char * makes p point to const char.

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