为什么C ++允许类型为std :: map而没有默认构造函数而不是std :: pair？

Question

检查以下C++代码：

#include <string>
#include <map>

class A
{
public:
    A (int a) {};
};

int main()
{

    std::map<std::string, A> p;
    return 0;
}

编译成功。 将std::map更改为std::pair ：

#include <string>
#include <utility>

class A
{
public:
    A (int a) {};
};

int main()
{

    std::pair<std::string, A> p;
    return 0;
}

编译器会抱怨：

$ clang++ test.cpp
In file included from test.cpp:1:
In file included from /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../include/c++/7.3.0/string:40:
In file included from /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/char_traits.h:39:
In file included from /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/stl_algobase.h:64:
/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/stl_pair.h:219:18: error: no matching
      constructor for initialization of 'A'
      : first(), second() { }
                 ^
test.cpp:13:31: note: in instantiation of member function 'std::pair<std::__cxx11::basic_string<char>, A>::pair'
      requested here
    std::pair<std::string, A> p;
                              ^
test.cpp:7:5: note: candidate constructor not viable: requires single argument 'a', but no arguments were provided
    A (int a) {};
    ^
test.cpp:4:7: note: candidate constructor (the implicit copy constructor) not viable: requires 1 argument, but 0 were
      provided
class A
      ^
1 error generated.

为什么C++允许类型为std::map而没有默认构造函数而不是std::pair ？

Answer 1

构造时std::map为空，这意味着它不需要构造A 另一方面， std::pair必须这样做才能完成初始化。

由于两者都是类模板，因此实际上只实例化了您使用的成员函数。 如果要查看预期的错误，则需要让映射尝试构造默认A ，例如：

int main()
{
    std::map<std::string, A> p;

    p[""];
}

Answer 2

差异是由于A不是默认构造的。 对案例立即选择它，因为它将尝试默认构造A实例。

最终地图案例将产生相同的错误。 如果你写p["Hello"];你可以看到这个p["Hello"]; ：

int main()
{
    std::map<std::string, A> p;
    p["Hello"]; // oops - default constructor required here for `A`
}