linux编译警告 will be initialized after

这个warning是由于初始化顺序引起的, 

// 会出现warning 的代码！
class cInit
{
public:
    cInit(std::string str, int a) : str_(str), a_(a)
    { }

private:
    int a_;
    std::string str_;
};

// OK
class cInit
{
public:
    cInit(std::string str, int a) : str_(str), a_(a)
    { }

private:
    std::string str_;
    int a_;
};