模板预编译 - C++模板按unsigned值编译

Question

#include <cstdio>

template<unsigned N>
int func() {
    if(N>100) {
        return 1;
    } else {
        static const char c = N; // g++: error, clang++: pass
        static const char s[1] = {N}; // both error
        printf("%c\n",c);
        return 0;
    }
}

int main() {
    func<1000>();
    return 0;
}

如图程序，为什么会有编译时错误呢？明明走了另一个分支啊！

Answer 1

In this case, if is not used, but specialization is used. The syntax that is logically equivalent to your code is:

template<unsigned N>
std::enable_if<(N <= 100), int>::type
func() {
    static const char c = N; // g++: error, clang++: pass
    static const char s[1] = {N}; // both error
    printf("%c\n",c);
    return 0;
}

template<unsigned N>
std::enable_if<(N > 100), int>::type
func() {
    return 1;
}

This will prevent the compiler from adding code containing syntax errors to the compilation during overload resolution.