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。当将 std::map
更改为 std::pair
时:
#include <string>
#include <utility>
class A
{
public:
A (int a) {};
};
int main()
{
std::pair<std::string, A> p;
return 0;
}
编译器会报错:
$ 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 std::map
的类型没有默认构造函数,而 std::pair
则不允许?
最佳答案
std::map
在构造时为空,这意味着它还不需要构造 A
。另一方面,std::pair
必须这样做才能完成其初始化。
由于两者都是类模板,所以只有你使用的成员函数才真正被实例化。如果你想看到你期望的错误,你需要让 map 尝试构造一个默认的 A
,例如:
int main()
{
std::map<std::string, A> p;
p[""];
}
关于c++ - 为什么 C++ 允许 std::map 的类型没有默认构造函数,而 std::pair 则不允许?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/49231881/