Pass class object as constructor parameter in C++

Question

Good afternoon, you guys! I’m studying for a C++ test I’m doing this week, and I’m having a hard time passing an object as a constructor parameter. Could you explain to me the right way to do it?

I have the following code:

#include <iostream>
#include <string>

using namespace std;

class Data {
public:
    Data(int dia, int mes, int ano) {
        this->dia = dia;
        this->mes = mes;
        this->ano = ano;
    };
    void setData(int dia, int mes, int ano) {
        this->dia = dia;
        this->mes = mes;
        this->ano = ano;
    };

    string getData() {
        string d = to_string(this->dia);
        string m = to_string(this->mes);
        string a = to_string(this->ano);
        return d + "/" + m + "/" + a;
    }

private:
    int dia, mes, ano;
};


class Pessoa {
public:
    Pessoa(Data d) {
        this->data = d;
    };
private:
    Data data;
};


int main()
{
    Data data1(1, 2, 1995);
    Data *data2 = new Data(3,4,1996);

    cout << data1.getData() << endl;
    cout << data2->getData() << endl;

    //Pessoa p(data1);
    //Pessoa *p2 = new Pessoa(data2);
    return 0;
}

As possible see in the section below, I tried to create a constructor that would receive a Date and allocate it in the Data encapsulated field:

class Pessoa {
public:
    Pessoa(Data d) {
        this->data = d;
    };
private:
    Data data;
};

But when compiling, I have this problem:

||=== Build: Debug in Aprendizado1 (compiler: GNU GCC Compiler) ===|
D:\Lucas\DESENVOLVIMENTO\12 Semestre\Aprendizado1\main.cpp||In constructor 'Pessoa::Pessoa(Data)':|
D:\Lucas\DESENVOLVIMENTO\12 Semestre\Aprendizado1\main.cpp|33|error: no matching function for call to 'Data::Data()'|
D:\Lucas\DESENVOLVIMENTO\12 Semestre\Aprendizado1\main.cpp|8|note: candidate: Data::Data(int, int, int)|
D:\Lucas\DESENVOLVIMENTO\12 Semestre\Aprendizado1\main.cpp|8|note:   candidate expects 3 arguments, 0 provided|
D:\Lucas\DESENVOLVIMENTO\12 Semestre\Aprendizado1\main.cpp|6|note: candidate: constexpr Data::Data(const Data&)|
D:\Lucas\DESENVOLVIMENTO\12 Semestre\Aprendizado1\main.cpp|6|note:   candidate expects 1 argument, 0 provided|
D:\Lucas\DESENVOLVIMENTO\12 Semestre\Aprendizado1\main.cpp|6|note: candidate: constexpr Data::Data(Data&&)|
D:\Lucas\DESENVOLVIMENTO\12 Semestre\Aprendizado1\main.cpp|6|note:   candidate expects 1 argument, 0 provided|
||=== Build failed: 1 error(s), 0 warning(s) (0 minute(s), 0 second(s)) ===|

If I add an empty constructor (no parameters), it works, but I don’t think it’s the right solution. How would it be the right one? Thank you!

Answer 1

The Person class builder is "wrong". The way it is written, it tries to call the default constructor of the Data class, and only after, tries to copy the parameter d to the date member. Since there is no standard constructor for the Data class, compilation fails.

To correct this, it should be used constructor Members initializer list https://en.cppreference.com/w/cpp/language/initializer_list.

It would look something like this:

class Pessoa {
public:
    Pessoa(Data d) 
    : data(d) {};
private:
    Data data;
};

Answer 2

Missing copy constructor in class Data,

On that line you call a copy builder no longer exists:

this->data = d;

That’s why the compiler shows these errors

Here explains what a copy builder is and how to implement it What is the C++ copy builder for? How do I implement it?

You could pass a reference to the object, Using references to your class Pessoa would just be like this:

class Pessoa {
public:
    Pessoa(const Data& d) {
        this->data = d;
    };
private:
    const Data& data;
};