类型检查的编译时失败

Question

My understanding of template is that when I write void foo<T>(T x) {...} and call foo<int>(x); and foo<float>(x) would generate foo(int x) and foo(float x) .

What I want is to type checking before some comparision, but since the compiler generates two version of the function, the comparision part will failed in compiling time.

My code is

template <typename T>
      void print(const std::vector<std::vector<T>>& matrix) {
          std::cout << std::setprecision(3) << std::fixed;
          for (int j=0; j < matrix[0].size(); j++) {
              for (int i=0; i < matrix.size(); i++) {
                  // Fail on this line ↓
                  if ((std::is_floating_point<T>::value) &&
                          (matrix[i][j] == std::numeric_limits<float>::lowest())) {
                      std::cout << "✗ ";
                      continue;
                  }
                  std::cout << matrix[i][j] << " ";
              }
          }
          std::cout << "\n";
      }

On the othe file I called

util::print<float>(best_value);
util::print<Point>(best_policy);

Declaration

std::vector<std::vector<float>> best_value;
std::vector<std::vector<Point>> best_policy;

How should I fix that problem while keeping the print function and do not have to add the comparision between Point and float ?

Answer 1

只需将std::numeric_limits<float>::lowest()更改为std::numeric_limits<T>::lowest()

Answer 2

In c++17, you might use if constexpr for condition known at compile time:

template <typename T>
void print(const std::vector<std::vector<T>>& matrix) {
    std::cout << std::setprecision(3) << std::fixed;
    for (const auto& row : matrix) {
        for (const auto& e : row) {
            if constexpr (std::is_floating_point<T>::value)) {
                if (e == std::numeric_limits<float>::lowest())) { // T instead of float?
                    std::cout << "✗ ";
                    continue;
                }
            }
            std::cout << e << " ";
        }
        std::cout << "\n";
    }
}