c-resources/CPlusPlus20ForProgrammers-m.../examples/ch09/fig09_28/fig09_28.cpp

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// fig09_28.cpp
// Friends can access private members of a class.
#include <fmt/format.h>
#include <iostream>
#include "fmt/format.h" // In C++20, this will be #include <format>
// Count class definition
class Count {
friend void setX(Count& c, int value); // friend declaration
public:
int getX() const { return m_x; }
private:
int m_x{0};
};
// function modifyX can modify private data of Count
// because modifyX is declared as a friend of Count (line 8)
void modifyX(Count& c, int value) {
c.m_x = value; // allowed because modifyX is a friend of Count
}
int main() {
Count counter{}; // create Count object
std::cout << fmt::format("Initial counter.m_x: {}\n", counter.getX());
modifyX(counter, 8); // change x's value using a friend function
std::cout << fmt::format("counter.m_x after modifyX: {}\n",
counter.getX());
}
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