Why am I getting an error converting a ‘float**’ to ‘const float**’?

Question

I have a function that receives float** as an argument, and I tried to change it to take const float**.

The compiler (g++) didn't like it and issued :

invalid conversion from ‘float**’ to ‘const float**’

this makes no sense to me, I know (and verified) that I can pass char* to a function that takes const char*, so why not with const float**?

Answer 1

See Why am I getting an error converting a Foo** → const Foo**?

Because converting Foo** → const Foo** would be invalid and dangerous ... The reason the conversion from Foo** → const Foo** is dangerous is that it would let you silently and accidentally modify a const Foo object without a cast

It goes on to give an example of how such an implicit conversion could allow me one to modify a const object without a cast.

Answer 2

This is a very tricky restriction. It is related to the aliasing rules of the language. Take a look at what the standards say, because I have faced this once before:

(Page 61)

[Note: if a program could assign a pointer of type T** to a pointer of type const T** (that is, if line //1 below was allowed), a program could inadvertently modify a const object (as it is done on line //2). For example,

int main() {
  const char c = ’c’;
  char* pc;
  const char** pcc = &pc; //1: not allowed
  *pcc = &c;
  *pc = ’C’; //2: modifies a const object
}

—end note]

Answer 3

If you converted the parameter to const float** you could then store a const float* at the memory location where the parameter points to. But the calling function thinks that this memory location is supposed to contain a non-const float* and might later try to change this pointed-to float.

Therefore you cannot cast a float** to a const float**, it would allow you to store pointers to constants in locations where pointers to mutable values are expected.

For more details see the C++ FAQ Lite.