C++ passing by reference or by value?

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What's the difference between passing by reference vs. passing by value? 18 answers

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When passing a variable by reference, whatever changes you make to it in the function are reflected back in the calling function.

On the other hand, when you pass a variable by value, the changes made to it are local, and hence not reflected in the calling function.

For example,

#include "iostream"
using namespace std;

void function1(int &x, int y) // x passed by reference
{
    x+=y;
}

void function2(int x, int y) // x passed by value
{
    x+=y;
}

int main()
{
    int x=10;
    function1(x, 10);  // x is passed by reference (no & while calling)
    cout << x << endl; // 20
    function2(x, 1000);// x is passed by value
    cout << x << endl; // 20
}

Notice that whatever value of y you pass in the call to function2 does not make any difference to the second cout statement.

You do not decide whether to pass values or references in main . The function definition decides that for you. Irrespective of pass by value or pass by reference, the format of calling a function remains the same.

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void getCoefficients(double &a, double &b, double &c);

This says, "I take 3 parameters - all of the type double & (reference to a double). Reference is quite a lot confused with pointers, so I recommend you read this up first.

Let's call the a,b,c inside the main as main_method_vars.

When you call getCoefficients , whatever this function does to the passed variables inside it, is reflected on the main_method_vars. Actually, this method works with the main_method_vars.

If instead you have void getCoefficients(double a, double b, double c) , this means that whenever you call this method with the main_method_vars, this method will copy the values of a,b,c and work with new copies, instead of working with the original passed copies to it.

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void getCoefficients(double &a, double &b, double &c);
void solveQuadratic(double a, double b, double c, double &x1, double &x2);

例如，函数getCoefficients，变量a，b，c通过引用传递，因此如果三个变量的值在getCoefficients函数中更改，则主函数的值也将在主函数中更改。

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