Definition

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int a = 123;
    ^
    l-value

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// GetValue() is l-value now.
// GetValue() return a lvalue reference.
int& GetValue() {
    static int value = 10; // Must be a persistent location.
    return value;
}

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int a = 123;
        ^^^
        r-value

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// GetValue() is r-value
// It's return a temporary value 10
int GetValue() {
    return 10;
}

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void SetValue(int value) {
}

int main() {
    int i = 10;
    SetValue(i);  // OK, passing a l-value.
    SetValue(10); // OK, passing a r-value.
}

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void SetValue(int& value) { 
}

int main() {
    int i = 10;
    SetValue(i);  // OK, passing a l-value.
    SetValue(10); // ERROR!! Can't passing a r-value.
}

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void SetValue(const int& value) { 
}

int main() {
    int i = 10;
    SetValue(i);  // OK, passing a l-value.
    SetValue(10); // OK, compiler will create a temp l-value.
}

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void SetValue(const int&& value) { 
}

int main() {
    int i = 10;
    SetValue(i);  // ERROR!! Can't bound l-value to r-value reference
    SetValue(10); // OK, passing a r-value
}