CODEWITHSUNDEEP
c++
Programmer
Programmer

Reputation: 112

Cannot Convert from const int* to int*

I'm trying to compile the following piece of code, but I'm getting a C2440 (visual studio) error. I've tried looking at other resources for help, but I can't find a good explanation. 

int main()
{
    int a = 100;
    SomeFunction(&a);
}

void SomeFunction(const int* value)
{
    //This line of code gives me the error. 
    int* variable = value;

    cout << "Value is " << *Variable << " end" << endl;
}

I know I can solve this problem by using int* variable = const_cast<int*> (value);, but I still don't understand why the above code is causing a problem.

Upvotes: 0

Views: 23287

Answers (4)

OldSchool
OldSchool

Reputation: 2183

const int * means that the function guarantees that value at this address will not change, but if you can do: 

int * variable = value;

Then you can also do

*variable=30;

In doing so, the guarantee of function that is of const pointer will be harmed.

Upvotes: 1

Mike Seymour
Mike Seymour

Reputation: 254461

The error's quite clear - a pointer conversion can't remove a const qualifier; otherwise, you could violate the constness:

int* variable = value; // Not allowed - but if it were...
*variable = 42;        // BOOM! changed a constant.

If you don't want to be able to change the value being pointed at, then keep it const

const int* variable = value;

If you do want to change it, then don't make it const in the first place:

void SomeFunction(int* value)

I know I can solve this problem by using const_cast

That's a bad idea - you'll get undefined behaviour if you abuse const_cast and try to modify a constant object. You should use const where you can, but not when you want to modify something.

Upvotes: 18

Ben Voigt
Ben Voigt

Reputation: 283644

The const int* means that you have the address of an int that you are not allowed to change.

An int* can be used to change the int it points to. Clearly that violates the above statement that you aren't allowed to change it.

const_cast does not actually "solve the problem". It lets you request to change something which cannot be changed. Such an attempt can cause your program to fail in uncontrolled ways (formally, undefined behavior). In your particular example, the variable a in main is not const, and so const_cast will successfully change it. But that creates close coupling and contradicts the promise made by a function whose signature is const int*.

Upvotes: 12

Sunil Bojanapally
Sunil Bojanapally

Reputation: 12658

int* variable = value; is wrong.

It should be,

int variable = *value

and

cout << "Value is " << variable << " end" << endl;

Upvotes: 2

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