Structs in C++ can have member functions just like classes, so it is incorrect to
say that structs cannot use functions.

For example, in the struct example I provided before:

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struct Point {
    double x;
    double y;
};
You can add functions to it like this :

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struct Point {
    double x;
    double y;
    void print() {
        std::cout << "x = " << x << ", y = " << y << std::endl;
    }
};
You can call the function like this:

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Point p;
p.x = 5.0;
p.y = 3.0;
p.print();

It will print "x = 5, y = 3"

In C++, structs are similar to classes in many ways, and they can have member 
variables and member functions just like classes. By default, the members of a
struct are public, while the members of a class are private, but you can use the
public, private, and protected access specifiers to control the access level of
struct members just like you would do for class members.

The main difference between structs and classes is the default access level for 
their members and the convention that structs are used for simple data structures
and classes for more complex types with data and behavior.

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On the other hand, a class is also a user-defined type that groups related data
together, but by default, all its members are private. Here is an example of a 
class:

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class MyClass {
    private:
        int x;
    public:
        int y;
        void setX(int val) {
            x = val;
        }
};
This class defines a type MyClass that has two members, x and y, where x is private
and y is public. Since classes have private members by default, you can't access
the private members directly, you have to use a public member function or method
to do so.

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MyClass obj;
obj.y = 10;
obj.setX(5);

In general, structs are used for simple data structures, while classes are used 
for more complex types that have both data and behavior. However, in practice, 
the distinction between structs and classes is not always clear-cut, and the choice
between them depends on the specific needs of the program.

It's worth noting that C++11 introduced the class keyword as a synonym for struct,
this means you can use class to define a struct with public members, or struct
to define a class with private members, but the convention is to use struct for
data structures and class for classes, and it's always a good idea to follow 
conventions in order to make your code more readable and maintainable.