How do I access boolean array from another function? c++

Question

I have a problem. I need to create a code where I can access bool array information from another function, edit the array and then send it back. I need to use a variable as the size of the array.

Global variable is not an option.

I've tried to pass it by a reference and also using structs.

code for example:

void x(bool (&reserved[sizeOfArray)) {
    if (reserved[1] == true) {
          cout << "it's true";

}    

main() {

int sizeOfArray = 6;
bool reserved[sizeOfArray];

x(reserved[sizeOfArray];

edit: the size of the array is determined when the program is already running

If the array size needs to be a variable then your best option is std::vector, variable length arrays are not supported in C++ — Alan Birtles
– Alan Birtles, Commented Dec 15, 2021 at 13:34
The name of the array is "reserved", not "reserved[sizeOfArray]". reserved[sizeOfArray] would be a bool, if it existed. — molbdnilo
– molbdnilo, Commented Dec 15, 2021 at 13:35
If you want to use the most readable C++ syntax : std::array<bool,6> reserved; void x(std::array<bool,6>& reserved){} — Pepijn Kramer
– Pepijn Kramer, Commented Dec 15, 2021 at 13:53
Have you tried accessing it via pointer. You can also pass array size as another function argument. — Abhinaya P
– Abhinaya P, Commented Dec 15, 2021 at 13:58

molbdnilo · Accepted Answer · 2021-12-15 13:44:39Z

1

If the size is only determined at runtime, you essentially have two options:

Use a dynamically-sized container, like std::vector

void x(std::vector<bool>& reserved)
Use the "C method" of passing a pointer to the array's first element along with the array size
void x(bool reserved[], size_t size)

The possible third option of having a "sentinel" value last in the array (like C-strings) won't work with bool, since you only have two values to choose from.

answered Dec 15, 2021 at 13:44

molbdnilo

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Comments

vikram · Accepted Answer · 2021-12-15 13:56:54Z

1

I think the lightweight method would be to use std::span instead.

#include <iostream>
#include <span>

void x(std::span<bool> reserved) {
    reserved[1] = false;
}    

int main() {

    constexpr size_t sizeOfArray = 6;
    bool reserved[sizeOfArray]{false, true};

    x(reserved);

    if (reserved[1] == false) {
        std::cout << "Hello!" << std::endl;
    }
}

answered Dec 15, 2021 at 13:56

vikram

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Comments

Ted Lyngmo · Accepted Answer · 2021-12-15 14:01:38Z

0

You could make it a function template, with the array size as a template parameter:

#include <iostream>

template<size_t N>
void x(bool (&reserved)[N]) {

    // make sure that the array size is at least 2 to not get out of
    // bounds access in reserved[1]:

    if constexpr (N >= 2) {    
        if (reserved[1] == true) {
            std::cout << "it's true\n";
        }
    }
}    

int main() {
    constexpr size_t sizeOfArray = 6; // must be a constant expression
    
    bool reserved[sizeOfArray]{false,true}; // rest will be false

    x(reserved);
}

Output:

It's true

edited Dec 15, 2021 at 14:01

answered Dec 15, 2021 at 13:34

Ted Lyngmo

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4 Comments

vikram Over a year ago

How about using std::span instead?

Ted Lyngmo Over a year ago

@vikram Sure, that'd work too but the above is pretty close to what OP had in his/her code already - for better or for worse.

vikram Over a year ago

I agree with that and I like your answer. But std::span makes the code less daunting and clearer. Perhaps just a personal opinion.

Ted Lyngmo Over a year ago

@vikram I like the span approach too. Oh, you added an answer for it. Here, have an upvote :)

rturrado · Accepted Answer · 2021-12-15 15:23:59Z

0

You can use boost::dynamic_bitset for dynamic vectors of bools. [Demo]

#include <boost/dynamic_bitset.hpp>
#include <iostream>  // cout

void x(const boost::dynamic_bitset<>& reserved) {
    if (reserved[1] == true) {
          std::cout << "it's true\n\n";
    }
    for (int i{0}; i < reserved.size(); ++i)
    {
        std::cout << "Bit " << i << ": " << reserved[i] << "\n";
    }
}

int main() {
    int sizeOfArray = 6;
    boost::dynamic_bitset<> reserved(sizeOfArray, 0b111010);
    x(reserved);
}

answered Dec 15, 2021 at 15:23

rturrado

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