initialise const struct with const pointer

Question

I want to form a struct from const parameters passed to the function. As the parameters are const, i guess the struct has to be const too. However, it does not work with pointers.

The following code compiles (MinGW 4.9.2 32bit)

struct structType_t {
    int b;
};

void func1(const int b) {
    const structType_t s={b};  // invalid conversion from 'const int*' to 'int*' [-fpermissive]

    // do something with s;
}

but with pointers it doesn't:

struct structType_t {
    int* b;
};

void func1(const int* b) {
    const structType_t s={b};  // invalid conversion from 'const int*' to 'int*' [-fpermissive]

    // do something with s;
}

Why does the compiler try to cast away the const here? So how can i use a const pointer to initialise a const structure?

const int and int const is the same type, const int* and int* const are different types. — Bo Persson
– Bo Persson, Commented Nov 17, 2015 at 14:42
Even if i change the type of b to const int * const its not compiling. The only solution i found, was a const_cast, which i guess is totaly safe here. But i'd like to know, why i need const_cast with pointers, but not with interger. — Bil_Bomba
– Bil_Bomba, Commented Nov 17, 2015 at 15:20

nh_ · Accepted Answer · 2015-11-17 14:46:15Z

2

If you change your struct to hold a const int* you can use it to store the const int* passed to the function, regardless of whether your s is const or not.

struct structType_t {
    const int* b;
};

void func1(const int* b) {
     const structType_t s={b};
     // or 
     structType_t s2={b};

    // do something with s or s2 ...
}

answered Nov 17, 2015 at 14:46

nh_

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2 Comments

Bil_Bomba Over a year ago

I can not change the struct to hold a const. However, declaring a struct as const is making all the members const, right?

nh_ Over a year ago

Yes, it will make it hold a const pointer to an int (int* const) but not a pointer to a const int (const int*), which is what you want to store.

Simia_Dei · Accepted Answer · 2015-11-17 15:57:37Z

1

In the first case you are creating a copy of an int. Copy of const int does not have to be const so it works. In the second case you are creating copy of a pointer to const int and assigning it to a pointer to an int - this is not allowed and that's why it does not compile.

answered Nov 17, 2015 at 15:57

Simia_Dei

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