How to declare a vector in a hpp? - c++

Question

Hello I know that to declare a std::vector I have to do like this
std::vector<double> a(0);
But in my file it does not work. Here is my code :
main.cpp :
#include "test.hpp"
int main()
{
Test test;
return EXIT_SUCCESS;
}
test.hpp :
#ifndef DEF_TEST
#define DEF_TEST
#include <iostream>
#include <vector>
class Test
{
public:
Test();
private:
std::vector<double> a(0);
};
#endif
and this is test.cpp :
#include "test.hpp"
Test::Test()
{
a.push_back(2.3);
std::cout << a[0] << std::endl;
}
And the compiler told me :
In file included from main.cpp:1:0:
test.hpp:11:23: error: expected identifier before numeric constant
std::vector<double> a(0);
^
test.hpp:11:23: error: expected ‘,’ or ‘...’ before numeric constant
In file included from test.cpp:1:0:
test.hpp:11:23: error: expected identifier before numeric constant
std::vector<double> a(0);
^
test.hpp:11:23: error: expected ‘,’ or ‘...’ before numeric constant
test.cpp: In constructor ‘Test::Test()’:
test.cpp:5:1: error: ‘((Test*)this)->Test::a’ does not have class type
a.push_back(2.3);
^
test.cpp:6:17: error: invalid types ‘<unresolved overloaded function type>[int]’ for array subscript
std::cout << a[0] << std::endl;
Thank you for your help !

Answer 1

It seems to me that you can not use a constructor for a member declaration of a class; to use constructors for the vector in a class you must specify it in a constructor for the class, e.g.
class Test {
private:
vector<double> a;
public:
Test() : a(0) {;}
};

Answer 2

You can initialize a variable using the following syntax:
std::vector<double> a(0);
but you can't use that for in-class initialization of class members. To initialize a member in-class, you can use the following syntax:
std::vector<double> a = {};

Answer 3

std::vector members will get initialized by the default initializer once an instance of your class is created.
If you want to call the vector initializer explicitly you can do it this way in your Test.cpp file:
Test::Test():
a(0) {
//...
}
Ps. An advantage of this is that you also can initialize constants that are members of your class.

Answer 4

You must do the following to initialize the vector as a class member:
class Test{
public:
Test();
private:
std::vector<double> a = std::vector<double>(0);
};
FYI, this code sizes the vector to 0, which is redundant. You can simply write std::vector<double> a and its size will be 0 to begin with. In other cases, if you want the vector to be of size n, then you use the method I've written with n instead of 0.

Answer 5

You can use an initialiser list for this after your constructer
Test::Test() : a(0)
{
...
}
You need to remove the (0) from your .hpp file.