constexpr1.cpp

gistfile1.cpp

#include <numeric>

constexpr int naiveSum(unsigned int n) {
    auto p = new int[n];
    std::iota(p, p+n, 1);
    auto tmp = std::accumulate(p, p+n, 0);
    delete[] p;
  // 如果这里注释掉delete，会报错
  // error: '(naiveSum(10) == smartSum(10))' is not a constant expression because allocated storage has not been deallocated
  //  4 |     auto p = new int[n]; 
  // 如果改成delete p也会报错
  // error: non-array deallocation of object allocated with array allocation
    return tmp;
}

constexpr int smartSum(unsigned int n) {
    return (n*(1+n))/2;
}

int main() {
    static_assert(naiveSum(10) == smartSum(10));        
    static_assert(naiveSum(11) == smartSum(11));
    return 0;
}