c++ - 构建成对 vector 时出现编译器错误

Question

请有人帮助解释为什么在 OS X 上使用 Xcode 5.1 编译以下代码时出现错误。Apple LLVM 版本 5.1 (clang-503.0.40)(基于 LLVM 3.4svn)。

我想在下面构造 X，向它传递一个对 vector 。

#include <iostream>
#include <string>
#include <vector>
#include <utility>

struct X
{
public:
    typedef std::vector<std::pair<std::string, std::string>> VectorType;

    X(VectorType& params) : m_params(params)
    {
    }

    VectorType m_params;
};

int main(int argc, const char * argv[])
{
    X::VectorType pairs
    {
        { "param-1", "some-string-1"},    // pair 0
        { "param-2", "some-string-2"},    // pair 1
        { "param-3", "some-string-3"},    // pair 2
        { "param-4", "some-string-4"},    // pair 3
        { "param-5", "some-string-5"},    // pair 4
        { "param-6", "some-string-6"},    // pair 5
        { "param-7", "some-string-7"}     // pair 6
    };

    X x
    {
        {pairs[0], pairs[2], pairs[5]}
    };

    return 0;
}

报错是:

/main.cpp:37:7: error: no matching constructor for initialization of 'X'
    X x
      ^
/main.cpp:6:8: note: candidate constructor (the implicit move constructor) not viable: cannot convert initializer list argument to 'X'
struct X
       ^
/main.cpp:6:8: note: candidate constructor (the implicit copy constructor) not viable: cannot convert initializer list argument to 'const X'
struct X
       ^
/main.cpp:11:5: note: candidate constructor not viable: cannot convert initializer list argument to 'VectorType &' (aka 'vector<std::pair<std::string, std::string> > &')
    X(VectorType& params) : m_params(params)
    ^
1 error generated.

Answer 1

你的构造函数应该通过 const 引用来获取它的参数

X(VectorType const & params)
             ^^^^^

否则，您无法传递临时 vector (正如您尝试做的那样)，因为临时 vector 无法绑定(bind)到非常量 lvalue 引用。