Thread: Can`t cast float* to void* pointers

Originally Posted by heatblazer

Can`t cast float* to void* pointers

This would be most unlikely: there is an implicit conversion from float* to void*. But now let's examine the error message closely:

Code:

main.cpp:227: error: invalid cast from type 'float' to type 'float*'

So nope, the problem is that you are trying to cast a float to a float*. Let's look at the code:

Code:

v->callbacks->addElement(v, (char*)"some long string");
v->callbacks->addElement(v, (int*) 10000);
v->callbacks->addElement(v, (long*) 29999999);
v->callbacks->addElement(v, (float*)3.15f);

There's no problem converting a string literal to char* as an array of char would be converted to a pointer to its first element anyway. The only worry is that you are storing a pointer to non-const void, which leaves open the question of whether what the pointer points to might be modified. That would be a Bad Thing since you are dealing with a string literal.

The next three lines are problematic: you probably want to store a pointer to an object with these values, but for the first two, you end up storing a pointer that has the value of those values, i.e., you are treating 10000 and 29999999 as addresses, but they could well be invalid. The third line is what caused the error that you observed, but it is no less problematic as the other two.

What you probably should have done is something like this:

Code:

char string_value[] = "some long string";
int int_value = 10000;
long long_value = 29999999;
float float_value = 3.15f;

v->callbacks->addElement(v, string_value);
v->callbacks->addElement(v, &int_value);
v->callbacks->addElement(v, &long_value);
v->callbacks->addElement(v, &float_value);

By the way, please note:

Originally Posted by C11 Clause 7.1.3 Paragraph 1 (part)

— All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.
— All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces.

Hence, your declaration of a typedef named _addElement and your declaration of a function named __addElement result in undefined behaviour.

Originally Posted by heatblazer

Yes... Thank you. So using constants directly is forbidden....

Not forbidden, just not what you're expecting.

A pointer, when you get right down to it, is an address. You can pass a literal address in, but unless you somehow know the literal address that contains the data you want to access, you probably need to create a variable and take its address at runtime.

(There are cases where you genuinely might have literal addresses to deal with. Systems with memory-mapped I/O, for example.)

Here, you're trying to pass a pointer - an address where some data lives. In the string case, all string literals will be loaded somewhere in memory already, that's how the program can access them. That's why it works - the string already has some space allocated for it to live. In the more general case, you first have to allocate space to store the data you want (e.g. by creating a variable of that type, or by otherwise dynamically allocating the space) before there is an address to pass to your function.