Memset function and bool arrays

Question

I was trying to learn about memset function and I read an article that we cannot initialise int array to 1 using memset. But I have declared a bool array and initialised it to true using memset function.

On printing the array elements the array elements are 1 1 1 rather than true. If initialised to false it gives 0 0 0. Also I declared an int variable x and set its value to a[0]+a[1] and on printing x it gives me 2. I am using c++ on devc++. Can anyone explain me the reason?

bool a[3]
memset( a,true,3*sizeof(bool))
cout << a[i]; // 1 1 1
int x = a[0] + a[1]; // 2

Welcome to StackOverflow! Please note that C and C++ are two different languages (for example, there is no `bool` data type in C), and if your question is about C++, please tag it with C++ instead of C. You can use the "edit" link below your question to fix that. — Heinzi, Oct 20 '21 at 07:05
Welcome to SO. There is no array if `int` involved in your code. Whatever bit patters is present in a `bool` variable, it will be evaluated as `0` or `1` even if the binary value is `255` — Gerhardh, Oct 20 '21 at 07:07
A little anecdote regarding the datatype of 'Bool' and why 'initialization' is important: I once had a fragment of code, that checked an (uninitialized) bool. Once with `!variable` and, in the else part, against `== true`. Neither branch triggered and the debugger later told me, that my bool had a value of 42. I had found my answer. — Refugnic Eternium, Oct 20 '21 at 07:21
_"we cannot initialise int array to 1 using memset"_: I hope you understand the reason. — Jabberwocky, Oct 20 '21 at 07:23
In C++ use [`std::uninitialized_fill`](https://en.cppreference.com/w/cpp/memory/uninitialized_fill). Smart implementations will use `memset` when it's appropriate. — Pete Becker, Oct 20 '21 at 14:28

score 2 · Answer 1 · answered Oct 20 '21 at 07:17

sizeof( bool ) in C++ or sizeof( _Bool ) in C are implementation defined but usually they are equal to 1 that is to the value of sizeof( char ) and the value true is stored as 1 and the value false is stored as 0.

For example (the C Standard, 6.2.5 Types)

2 An object declared as type _Bool is large enough to store the values 0 and 1

So this call of memset

memset( a,true,3*sizeof(bool));

set all elements of the array a to 1.

This statement

cout « a[i];

outputs boolean values equal to true as integer constants 1. If you want to output as a string you need to use the manipulator std::boolalpha. For example

cout « std::boolalpha << a[i];

In this declaration

int x = a[0] + a[1];

the operand a[2] and a[1] yield the value 1. So their sum is equal to 2.

Jerry Coffin · Answer 2 · 2021-10-20T07:31:17.327

When you print out your bools, the stream has a boolalpha flag to determine whether they print out as true/false or as 1/0. If you want them to print out as true/false, you can do something like:

std::cout << std::boolalpha << a[i];

I wrote an answer some time ago that goes into more detail about this: https://stackoverflow.com/a/15960054/179910

For the sake of compatibility with C, C++ defined implicit conversions from int to bool and bool to int, so 0 converts to false and any other value to true, while false converts to 0 and true converts to 1. So when you do a[2] + a[1], it converts each bool to the corresponding int, then adds those together. Not always the behavior you'd want, but something that's used in enough C code that they apparently thought it necessary to maintain anyway (and I think they were right).

Getting back to memset, the problem with setting ints to non-zero values is that memset always treats the data you give it as an array of char, regardless of the type of data that's actually there. Each char (by definition) occupies a single byte. For example, let's assume you have 4-byte ints on your implementation. In this case, if you use memset to set those ints to 1, what you're going to get is each int set to the value 0x01010101 (which in decimal will be 16,843,009).

In C++, std::fill or std::fill_n tends to work out better as a rule. It's implemented as a template so its behavior is tailored to the type being written, so if you use it to fill an array of int with 1, each int will actually have the value 1.

Memset function and bool arrays

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