多类型std::initializer_list

Question

我和std::initializer_list有问题。我把它简化为一个简单的例子：

#include <initializer_list>
#include <cstdio>


class Test {
    public:
        template <typename type> Test(const std::initializer_list<type>& args) {}
};

int main(int argc, char* argv[]) {
    Test({1,2});

    getchar();
    return 0;
}

当使用g++ test_initializer.cpp -std=c++0x编译时，它编译和运行良好。但是，如果将第11行更改为Test({1,2.0});，则可以获得：

ian@<host>:~/Desktop$ g++ test_initializer.cpp -std=c++0x
test_initializer.cpp: In function ‘int main(int, char**)’:
test_initializer.cpp:11:14: error: no matching function for call to ‘Test::Test(<brace-enclosed initializer list>)’
test_initializer.cpp:11:14: note: candidates are:
test_initializer.cpp:7:28: note: template<class type> Test::Test(const std::initializer_list<_Tp>&)
test_initializer.cpp:5:7: note: constexpr Test::Test(const Test&)
test_initializer.cpp:5:7: note:   no known conversion for argument 1 from ‘<brace-enclosed initializer list>’ to ‘const Test&’
test_initializer.cpp:5:7: note: constexpr Test::Test(Test&&)
test_initializer.cpp:5:7: note:   no known conversion for argument 1 from ‘<brace-enclosed initializer list>’ to ‘Test&&’

我怀疑这是因为编译器无法确定生成初始化程序列表的类型。是否有一种方法可以修复这个示例，以便它可以处理不同的类型(并且仍然使用初始化程序列表)？

Answer 1

std::initializer_list只需要一种类型。如果需要不同的类型，可以使用各种模板：

template<typename... Args>
Test(Args&&... args);

/* ... */

int main()
{
    Test(1, 2.0);
}

Answer 2

让initializer_list保存最任意的指针( void* )，然后在那里进行自己的转换。下面是一个例子。

#include <initializer_list>
#include <iostream>

using std::initializer_list;
using std::cout;
using std::endl;

class Person {
    private:
        string _name;
        int _age;
    public:
        Person(initializer_list<void*> init_list) {
            auto it = init_list.begin();
            _name = *((string*)(*it));
            it++;
            _age = *((int*)(*it));
        }
        void print() {
            cout << "name: " << _name << ". age: " << _age << endl;
        }
};

int main(void) {
    string name{"Vanderbutenburg};
    int age{23};
    Person p{&name,&age};
    p.print(); // "name: Vanderbutenburg. age: 23"

    return 0;
}