Const gets var, I can't pass it to the template

Question

What I want to do:

int const bitsPerInt = log2(X);
bitset<bitsPerInt>  bits(a random number...);

but I get this error:

'bitsPerInt' cannot appear in error constant expression: template argument 1 is not valid

Answer 1

If you really need to do this, create your own log2, which works at compile time and passes it to the bitter template argument.

constexpr unsigned Log2(unsigned n, unsigned p = 0) {
    return (n <= 1) ? p : Log2(n / 2, p + 1);
}

constexpr size_t bitCount = Log2(X);
std::bitset<bitCount> bits;

Living example .

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Here's a solution using template metaprograms, i.e. without use constexpr:

template<int N,unsigned int P=0>
struct Log2 { enum { value = Log2<N/2,P+1>::value }; };

template <unsigned p>
struct Log2<0, p> { enum { value = p }; };

template <unsigned p>
struct Log2<1, p> { enum { value = p }; };

std::bitset<Log2<4>::value> bits;

Living example .

This version should work both in C ++ 03 and C ++ 11; however, if you have access to C ++ 11, I would still recommend the path constexpras it was cleaner (easier to understand).

Answer 2

( , , ) . ++. .