c++ - 短路模板实例化

Question

我的代码有如下布局

// A exists only as a true specialization
template<int Boolean>
struct A;

template<>
struct A<1> {};

// B exists only as a false specialization
template<int Boolean>
struct B;

template<>
struct B<0> {};

// then I use it like so
cond ? A<cond> : B<cond>; // actually I want to access an enumeration member value

短路不适用于三元运算符吗？为什么会出现编译错误？

Answer 1

三元运算符要求您知道表达式中使用的类型，以便形成通用类型。

在您的情况下，您可以使用类似的东西(C++11，参见 std::conditional):

#include <iostream>
#include <typeinfo>
using namespace std;

// A exists only as a true specialization
template<int Boolean>
struct A;

template<>
struct A<1> {
    void func() { cout << "A"; }
    };

// B exists only as a false specialization
template<int Boolean>
struct B;

template<>
struct B<0> {
    void func() { cout << "B"; }
};


int main() {

    const bool cond = true;
    // std::conditional can provide a compile-time ternary-like evaluation
    typedef std::conditional<cond ,A<cond>,B<cond>> myType;

    myType::type Obj;
    Obj.func();


    return 0;
}

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