C++: error: passing 'const X' as 'this' argument discards qualifiers [-fpermissive]

Meaning

Calling obj.method() on a const Obj& is only allowed if method() is marked const. Mark every method that does not change the object const; that is also what makes them usable from const references and in containers.

Common causes

• Member function that only reads data is not declared const
• Const reference or pointer parameter calling a non-const method
• Range-for over a const container using non-const methods
• operator[] or begin() without a const overload

Detailed fix by platform

Code examples

struct Stack {
    int top() const { return v[n - 1]; }   // const: only reads
    void push(int x) { v[n++] = x; }       // modifies, so non-const
    int v[16]; int n = 0;
};

void print(const Stack& s) {
    s.top();          // OK
    // s.push(1);     // error: passing 'const Stack' as 'this' argument discards qualifiers
}

How to diagnose

1. Object — Is the object const?
2. Method — Is the called method declared const?
3. Intent — Does the method modify state?

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