CODEWITHSUNDEEP
c++gccconstantsconstant-expression
hu wang
hu wang

Reputation: 421

Why can't this const argument match a non-type template parameter?

When compiling the code below with gcc, I get an error: 'i' cannot appear in a constant-expression.

Why is this?

#include <iostream>

using namespace std;
template<int p>
class C
{
public:
    void test();
};

template<int p>
void C<p>::test()
{
    p = 0;
}

char const *const p = "hello";
int main()
{
    const int i = (int)p;
    C<i> c;
}

Upvotes: 0

Views: 111

Answers (1)

R. Martinho Fernandes
R. Martinho Fernandes

Reputation: 234474

The variable i is not mutable at runtime because it is const, but it is not a "constant expression" because it is not evaluated at compile-time.

(int)p; is a reinterpret_cast. Integral constant expressions cannot have reinterpret_cast. It is explicitly forbidden (§5.19/2):

A conditional-expression is a core constant expression unless it involves one of the following as a potentially evaluated subexpression (§3.2), but subexpressions of logical AND (§5.14), logical OR (§5.15), and conditional (§5.16) operations that are not evaluated are not considered [Note: An overloaded operator invokes a function.—end note ]:

— [...]

— a reinterpret_cast (§5.2.10);

— [...]

Upvotes: 2

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