C++ returning wrong boolean values when in function

Question

I'm writing a function to check if all values of an array are true. If I write

int main(){
bool Janmaat[3]={true, true, true};

int size_arr=sizeof(Janmaat)/sizeof(Janmaat[0]);
bool value=true;
int i;

for(i=0;i<size_arr;i++){
    if(!Janmaat[i]){
        value=false;
    }
}
cout<<value;
return 0;
}

value stays true. If I encapsulate it into a function though

bool all_true(bool arr[]){

int size_arr=sizeof(arr)/sizeof(arr[0]);
bool value=true;
int i;

for(i=0;i<size_arr;i++){
    if(!arr[i]){
        value=false;
    }
}

return value;

}

int main(){

bool Janmaat[3]={true, true, true};

cout<<all_true(Janmaat);
return 0;
}

it returns false. What's going on?

cadaniluk · Answer 1 · 2015-12-23T19:32:26.720

3

When an array is passed to a function expecting a pointer, it automatically decays to a pointer. That means that arrays cannot be passed by value. arr from all_true is a pointer. sizeof(arr) will therefore yield the size of the pointer, not the whole array.

Different ways to handle this:

Pass the size as an argument. Chang the declaration to
```
bool all_true(bool arr[], size_t size)
```
and adjust the code accordingly.
Pass an std::array or a std::vector (std::bitset is technically feasible too). As this is C++, this is surely the better option.

Notes:

If one element is false, all_true will definitely return false. To shorten the runtime you could return false; in the if statement immediately.

Y'know Linus Torvalds? The guy who invented Linux and regularly insults Linux kernel "developers" (some deserve the double quotes; some don't)?
This might interest you then. :-)

edited Dec 23 '15 at 19:32

answered Dec 23 '15 at 19:15

cadaniluk

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1

It decays to pointer if the function parameter is a pointer. It doesn't decay to pointer when passed to any old function. – juanchopanza Dec 23 '15 at 19:16
@cad So is the only way to declare the size of the array? – Plinth Dec 23 '15 at 19:16
@Plinth I added something. – cadaniluk Dec 23 '15 at 19:17
1

@Plinth You don't need to. `all_of(begin(Janmaat), end(Janmaat), [](bool b) { return b; });` – juanchopanza Dec 23 '15 at 19:17
1

3. Make the function a template and accept the array by reference so it doesn't decay. – Lightness Races in Orbit Dec 23 '15 at 19:19
@juanchopanza Regarding your first comment, I added something in the first paragraph. Anyway, whadduya mean with "any **old** function"? – cadaniluk Dec 23 '15 at 19:21
@cad I mean any type of function. – juanchopanza Dec 23 '15 at 19:21
@juanchopanza That's probably a lot easier. Thanks! – Plinth Dec 23 '15 at 19:30

C++ returning wrong boolean values when in function

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