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

Check the following C++ code:

#include <string>
#include <map>

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

int main()
{

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

The compilation is successful. While change std::map to std::pair :

#include <string>
#include <utility>

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

int main()
{

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

The compiler will complain:

$ 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.

Why does C++ allow type has no-default constructor for std::map while not std::pair ?

Answer 1

std::map is empty when constructed, which means that it does not need to construct an A just yet. The std::pair , on the other hand, has to do it in order to complete its initialization.

Since both are class templates, only the member functions you use are actually instantiated. If you want to see the error you expect, you need to have the map try to construct a default A , for example:

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

    p[""];
}

Answer 2

The difference is due to A not being default-constructible. The pair case picks this up immediately as it will attempt to default-construct an A instance.

Eventually the map case will yield the same error. You can see this if you write p["Hello"]; :

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