C++ vector instance is operational when function returns it

In my class, I have two vector objects:

std::vector<std::string> vecteur_noms_A_;
std::vector<std::string> vecteur_noms_B_;


I also have the following function:

std::vector<std::string> & get_vecteur_noms_A_()
return vecteur_noms_A_;



but I do not have the same for the B one!

B

When I instantiate my class, and do the following instructions in my class constructor; I can see (via debugging or when It throws an error) that:

vecteur_noms_A_.push_back("some value"); // is ok

vecteur_noms_B_.push_back("some other value"); // is not ok <- throws an error.


Debugging let me see that an object (or instance ?)(empty vector) is existing for vecteur_noms_A_ but not for vecteur_noms_B_.

vecteur_noms_A_

vecteur_noms_B_

It is not clear why I observe that comportment I did not expect, I would be happy if I get an explanation.

Does the fact that I define a function forces the compiler to instanciate the object?

Here is an example :

#include <iostream>
#include <vector>

using namespace std;
class GP

public:
//Constructeur
GP(std::vector<std::string> list_names_A, std::vector<std::string> list_names_B
);

//Fonctions
void construction_vecteur_A_B_(std::vector<std::string> list_names_A, std::vector<std::string> list_names_B);


std::vector<std::string>& get_vecteur_noms_A()
return vecteur_noms_A_;


std::vector<std::string> vecteur_noms_A_;
std::vector<std::string> vecteur_noms_B_;
;



GP::GP(std::vector<std::string> list_names_a, std::vector<std::string> list_names_b)

construction_vecteur_A_B_(list_names_a, list_names_b);




void GP::construction_vecteur_A_B_(std::vector<std::string> list_names_a, std::vector<std::string> list_names_b)

for (int i = 0; i < list_names_a.size(); i++)
vecteur_noms_A_.push_back(list_names_a[i]);

for (int i = 0; i < list_names_b.size(); i++)
vecteur_noms_B_.push_back(list_names_b[i]);




int main()


std::vector<std::string> list_names_A = std::vector<std::string>();
std::vector<std::string> list_names_B = std::vector<std::string>();
list_names_A.push_back("A");
list_names_B.push_back("B");

GP(list_names_A, list_names_B);

return 0;



Since the program written here weems to throw no error, I would like to uderstand why :

vecteur_noms_A_


and

vecteur_noms_B_


are existing at execution so that I can do the pushback.
Why isn't it necessary to apply this to them in the constructor:

vecteur_noms_A_ = std::vector<std::string>();
vecteur_noms_B_ = std::vector<std::string>();


Thanks for your help.

1 Answer
1

With regards to your last question. It is not necessary to assign a value, such as std::vector<std::string>(), to the member vectors, vecteur_noms_A_, vecteur_noms_B_ as they are default constructed, when your class GP is used, before the body of the constructor is executed.

std::vector<std::string>()

vecteur_noms_A_

vecteur_noms_B_

GP

Notice therefore that if you were to do the following in the body of the constructor of GP:

GP

vecteur_noms_A_ = std::vector<std::string>();
vecteur_noms_B_ = std::vector<std::string>();


It would be equivalent to the following operation:

vecteur_noms_A_.operator=(std::vector<std::string>());
vecteur_noms_B_.operator=(std::vector<std::string>());


That is you are assigning an empty vector (using the copy assignment operator) to your already default constructed empty vector, which is redundant.

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