Why my random function always not undeclared? - c++

Question

I'm writing my code on linux . But g++ always tells me"Use of undeclared identifier 'random'".I don't know why I have declare it in "Myvector.h"
my code is like :
Myvector.h
class MyVector {
private:
std::vector<double> data;
const int N;
static bool _bDim;
public:
MyVector(); //默认初始化
MyVector(int a); //设置维度初始化
MyVector(std::initializer_list<double> list);
~MyVector();
double &operator[](int);
MyVector &operator=(const MyVector a) {
MyVector b(outN(a));
this->data = a.data;
return *this;
};
friend MyVector random(int a);
}
#endif // MYVECTOR_H_
Myvector.cpp
#include "Myvector.h"
#include <cstdlib>
#include <iostream>
#include <math.h>
#include <time.h>
using namespace std;
bool MyVector::_bDim = true;
MyVector::MyVector() : N(3) {
data = vector<double>(N, 0.0);
_bDim = false;
};
MyVector::MyVector(int a) : N(a) {
data = vector<double>(N, 0.0);
_bDim = false;
};
MyVector::MyVector(std::initializer_list<double> list) : N(list.size()) {
for (auto i = list.begin(); i != list.end(); i++) {
data.push_back(*i);
}
};
MyVector::~MyVector(){
};
double &MyVector::operator[](int i) { return data[i]; }
MyVector random(int a){
MyVector u(a);
srand(time(NULL));
for(int i=0;i<a;i++){
u[i]=rand();
}
return u;
}
main.cpp
#include "Myvector.h"
#include <iostream>
#include<cstdlib>
#include<math.h>
#include <time.h>
using namespace std;
int main(){
MyVector z=random(1);
return 0;}
In fact ,I just know nothing about it. Is there someone going to help me?Thank you.
Below is nothing meaningful. I just need more words to ask this problem.

Answer 1

In the main function of the main.cpp file the following function is called:
MyVector z=random(1);
This appears to be a function which takes a single int argument. Additionally, there is such a function defined in the Myvector.cpp but not declared in Myvector.h (i.e., the main.cpp file does not see any function declaration for the definition).
Update the Myvector.h header to declare the MyVector random(int a) function. Also, the friend declaration is for a random function with 2 parameters, which doesn't look right.

Answer 2

You have to declare the function random somewhere, e.g.
#include "Myvector.h"
#include <iostream>
#include<cstdlib>
#include<math.h>
#include <time.h>
using namespace std;
MyVector random(int);
int main(){
MyVector z=random(1);
return 0;
}

Answer 3

The problem is random in your main function. That one is not declared.
Declaring a friend function means that function has access to the class as if it were a method. It doesn't declare the function at any time, just allows it inside the class.

Answer 4

Your random function is defined is some header file you have included. In your error message you see the return type of random is long int. And you have declared it as MyVector. I am not sure if math.h or time.h have it.
Solution 1: Change the name of your function.
Solution 2: Put your function in a namespace in order to avoid name ambiguity.