CODEWITHSUNDEEP
c++pointersreferenceconstexpr
Alexey
Alexey

Reputation: 35

c++ constant pointer in the constexpr class

Good day! Please help me for my code, I try to make constexpr class with const pointer to non-constant and change non-constant variable in future, my compilator says

"error: ‘Actuator{const Pin{1ul, 1ul}, const Pin{1ul, 2ul}, const Pin{1ul, 3ul}, ((velocity_type*)(& velocity))}’ is not a constant expression",

Object act1 always life, because its code for ARM embedded device

Code:

#include <cstddef>

typedef std::size_t port_type;
typedef std::size_t pin_type;
typedef std::size_t velocity_type;

class Pin {
private:
    port_type const _port;
    pin_type const _pin;

public:
    constexpr Pin(port_type const port, pin_type const pin) :
            _port { port }, _pin { pin } {
    }
};

class Actuator {
private:
    Pin const _enable_pin;
    Pin const _dir_pin;
    Pin const _step_pin;
    velocity_type* const _velocity; //constant pointer to non-constant variable

public:
    constexpr Actuator(Pin const ep, Pin const dp, Pin const sp, const velocity_type velocity) :
            _enable_pin { ep }, _dir_pin { dp }, _step_pin { sp }, _velocity(const_cast<velocity_type*>(&velocity)) {
    }
    void set_velocity(const velocity_type velocity) const {*_velocity = velocity;} //try to change velocity
};

int main() {
    constexpr Actuator act1 ( Pin { 1, 1 }, Pin { 1, 2 }, Pin { 1, 3 }, 1u );
    act1.set_velocity(1u);
}

Upvotes: 1

Views: 150

Answers (1)

W.F.
W.F.

Reputation: 13988

According to c++ standard [expr-const]/2 (emphasis mine)

An expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine, would evaluate one of the following expressions:
(...)
15. a reinterpret_­cast

((velocity_type*)(& velocity)) is definitely a form of reinterpret casting so it cannot be used in constant expression...

Upvotes: 1

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