创建std :: vector <Class> 稍后调用Class构造函数

Question

I've been googling and stackoverflowing this question, but couldn't find any answer.

The thing I am trying to do is create a vector with a custom class, which I have no problem with. I understand I need to do:

std::vector<ExtClass> varName;

But, what I want to do is this:

Since varName is now of type ExtClass, I want to call it's constructor in another class. To clarify, I think the code will explain this the best:

class ExtClass {
    public:
        int ext;
        int pet;

    ExtClass();
};

ExtClass:ExtClass() {
    this->ext = std::rand();
    this->pet = std::rand();
}

________________________

class MainClass {
    public:
        std::vector<ExtClass> attribute;

    MainClass(int);
}

MainClass(int size) {
    this->attribute.reserve(size) // reserving enough places in vector
    for (int i = 0; i < size; ++i)
        this->attribute[i] = new ExtClass(); // problem
}

I have used all public members to avoid unnecessary code.

I thought since std::vector attribute is a vector of ExtClass elements, and since none have been defined, that I could assign each element of the vector as:

new ExtClass()

and that would create, for example:

attribute[0].ext = 5125;
attribute[0].pet = 151;

This is definitely not the case and the compiler argues on this line with:

note: candidate: constexpr ExtClass& ExtClass::operator=(const ExtClass&)
class ExtClass {
      ^~~~~~
note:   no known conversion for argument 1 from ‘ExtClass*’ to ‘const ExtClass&’
note: candidate: constexpr ExtClass& ExtClass::operator=(ExtClass&&)
note:   no known conversion for argument 1 from ‘ExtClass*’ to ‘ExtClass&&’

I am not an expert in C++, but relatively new to it and I am practicing. Can you tell me what is my mistake? What assumption did I make wrong?

Thanks!

Answer 1

Code as written can be fixed by this:

MainClass(int size) 
   : attribute( size )
{
}

that's it it will initialize attribute with size default constructed objects of ExtClass .

Note that this code:

this->attribute.reserve(size) // reserving enough places in vector
for (int i = 0; i < size; ++i)
    this->attribute[i] = new ExtClass(); // problem

even if we fix it to be compilable:

this->attribute.reserve(size) // reserving enough places in vector
for (int i = 0; i < size; ++i)
    this->attribute[i] = ExtClass(); // problem

leads to UB as you try to access elements of empty vector - calling reserve does not change how many objects vector holds, only capacity for future use.

Answer 2

As the comments above indicate, you can replace:

for (int i = 0; i < size; ++i)
    this->attribute[i] = new ExtClass();

with:

for (int i = 0; i < size; ++i)
    this->attribute.emplace_back ();

Your vector will then manage the lifetimes of your ExtClass objects for you.