C ++什么是“从这里实例化”错误？

Question

I am new to C++ programming, I am wondering what is "instantiated from here" error?

struct Data {
    Data(int a, int b) {
        x = a;
        y = b;
    }
    int x;
    int y;
};

std::map<int, Data> m;
m[1] = Data(1, 2);

I got several error messages

• no matching function for call to "Data::Data()"
• "instantiated from here" error

Thanks.

Answer 1

There is no default constructor for struct Data . The map::operator[] returns a default constructed instance of its value type, in this case struct Data .

Either supply a default constructor:

Data() : x(0), y(0) {}

or use std::map::insert() :

m.insert(std::pair<int, Data>(1, Data(1, 2)));

Answer 2

C++ what is “instantiated from here” error?

That is not an error, but the continuation of the previous error adding extra information. The compiler is adding the error: prefix so that it is easier to read (or parse) what lines belong to the error.

You can read the whole block as a single error:

No matching function call to Data::Data() instantiated from...

Answer 3

您需要提供一个不带参数的构造函数。

Data::Data(){}

Answer 4

这意味着只有当编译器开始解析（ 实例化 ）模板std::map::map()时，错误（“无匹配函数调用”）才变得明显。

Answer 5

When you insert an item into a map using operator[] , what happens is that the map inserts a default-constructed object for the specified key to refer to, then the value you're assigning gets copied into that default constructed object.

To create that default-constructed object, a default constructor needs to be available. In your case, sine you've specified a constructor that takes two arguments (and not supplied default values for those arguments), the compiler will not automatically synthesize a default constructor for you.

Under the circumstances, I'd probably modify your constructor to something like this:

Data(int a=0, int b=0) : x(a), y(b) {}

Note that you should generally prefer initializing variables in an initialization list (if possible) over assigning to them in the body of the constructor.

Answer 6

no matching function for call to "Data::Data()"

You need to provide a default constructor. See the answer to this question:

Template Error: no appropriate default constructor available

"instantiated from here" error

You can only have variable declarations in the global area. You need do that assignment from within a function. Correct code:

#include <map>

struct Data {
    Data(int a, int b) {
        x = a;
        y = b;
    }
    int x;
    int y;

    Data() {};
};

std::map<int, Data> m;

void main()
{
   m[1] = Data(1, 2);
}