Error: cannot bind non-const lvalue reference of type ‘int&’ to an rvalue of type ‘int’

Question

I need to create a Bar object, which has a private object Foo f.

However, the value of Foo object parameter should be passed by the specific method int genValue().

If I initialize f in the constructor scope Bar(){...}, the compiler yell error, something like there is no constructor Foo().

If I construct like this Bar(): f(genValue()), the compiler yells the error:

test.cpp: In constructor ‘Bar::Bar()’:
test.cpp:16:19: error: cannot bind non-const lvalue reference of type ‘int&’ to an rvalue of type ‘int’
 Bar(): f(genValue()){    
            ~~~~~~~~^~
test.cpp:7:2: note:   initializing argument 1 of ‘Foo::Foo(int&)’    
 Foo(int &x) {    
 ^~~

Example code:

class Foo {
public:
    Foo(int &x) {
        this->x = x;
    }
private:
    int x;
};
class Bar {
public:
    Bar(): f(genValue()){
    }
private:
    Foo f;
    int genValue(){
        int x;
        // do something ...
        x = 1;
        return x;
    }
};
int main() {
    Bar bar ();
    return 0;
}

How can I fix the problem, if I don't want to modify Foo class and its argument value should be passed from genValue()? And, I don't want to use pure pointer (*), but a solution with smart pointer is okay!

Solution 1

A non-const reference parameter, such as an int&, can only refer to an "lvalue," which is a named variable.

auto takes_nonconst_reference = [](int&){};
auto takes_const_reference = [](const int&){};
auto takes_value = [](int){};
auto returns_int = []{return 42;};
int foo = 1;
// OK
takes_nonconst_reference(foo);
takes_const_reference(foo);
takes_const_reference(returns_int());
takes_value(foo);
takes_value(returns_int());
// compilation error, value returned from a function is not a named variable
takes_nonconst_reference(returns_int());

In this particular case, since your class is storing a copy of the constructor parameter, you should pass it by value (int, not int& nor const int&).

Solution 2

Don't pass int&, it can't be bound to a constant or temporary because those can't be modified - use const int& instead.

Actually for simple types you should prefer to pass by value instead, and let the optimizer worry about providing the best implementation.