Given an array map with size of 256, what is the best way to initialize it so that each element is false?
bool map[256];
for (int i=0; i<256; i++)
{
map[i] = false;
}
Thank you
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5 Answers
bool map[256] = { false };
edit: for a reference to the standard as to why this is legal, see the answers below.
3 Comments
q0987
Hello Idan, This is a C++ standard way to initialize all elements in an array? -- thank you
Idan K
yes, it is defined in the standard (trying to look it up as we speak)
Steve Jessop
I've looked it up, if you're still looking. See my answer for the paragraph number(s).
bool map[256] = {false};
(C++ also allows bool map[256] = {};. The above works in C too.)
This will set map[0] to false, and "default-initialize" the rest of 255 members (C++98 §8.5.1/7: "If there are fewer initializers in the list than there are members in the aggregate, then each member not explicitly initialized shall be default-initialized"; C99 §6.7.8/21).
For bool, "default-initialize" means set to (bool)0, i.e. false (C++98 §8.5/5; C99 §6.7.8/10).
Note that this method doesn't work if you want to initialize to true.
7 Comments
Alok Save
Just to mention, this is only valid for C++ but not for C.
q0987
Hello Als, thank you for reminder me that C doesn't have bool type.
eq-
@q0987: C has had a boolean type for 11 years now. For example C# didn't exist 11 years ago.
Steve Jessop
@KennyTM: I think value initialization was added in TC1/C++03, and in that standard the additional members are value-initialized, not default-initialized. Not that it makes any difference in this case. If you know of a compiler that implements C++98 but not C++03, please take it out and shoot it ;-)
jgottula
+1 for mentioning that it will not work correctly for
true. |
Like this:
bool map[256] = { 0 };
Some would say this:
bool map[256] = {};
But to me that just looks a little weird.
Why use "0" rather than "false"? It's not a big deal, but I do it because the array initializer is in effect padded out to the size of the array with 0, not with copies of the first element. Since the initialization of elements 1 through 255 is defined in terms of the value "0" (which of course converts to false), I see no harm in element 0 being initialized with "0" too. Having a "0" at the end of a too-short initializer list for scalar types reminds me of what's really going on.
Specifically, the standard defines this at 8.5.1/7 (initializer list shorter than array), and 8.5/5 (value-initializing a bool is equivalent to initializing it with the value 0 converted to bool).
Comments
Absent a reason to do otherwise, the best choice is probably to use a vector:
std::vector<bool> map(256, false);
vector<bool>, however, is a specialization that's rather different from what you might expect. Depending on the situation, you might prefer:
std::vector<char> map(256, false);
Comments
In C++, X = { 0 } is an idiomatic universal zero-initializer. This was a feature carried forward from C.
bool map[256] = { 0 };
1 Comment
Steve Jessop
If you want to be really pedantic, in C++ it constructs the first element with
0, and value-constructs the other elements. For a scalar type like bool, this all amounts to zero-initialization (which is why it's just pedantry in this case), but for class types it might be different.