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

c++ const-correctness
Omry Yadan picture Omry Yadan · Mar 17, 2010 · Viewed 12.8k times · Source

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

AraK picture AraK · Mar 17, 2010

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]

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