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Calin Baenen
Calin Baenen

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Why do integral types get weirdly initialized inside of std::array in C++, and how can I fix it?

This was tested in SoloLearn's C++ Playground, I'm making the assumption my compiler will do something similar to the one SL uses.

So, I'm playing around with std::array<T, s>, because I plan on using it in my game.

When I do std::array<bool, 5> def; and print out the items in each index, I get:

Default 0: 0
Default 1: 0
Default 2: 0
Default 3: 0
Default 4: 0
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When I do std::array<std::string, 5> def;:

Default 0: 
Default 1: 
Default 2: 
Default 3: 
Default 4: 
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however, when I use an integral type like int, I get numbers that are totally random in some of the spots. - Only a few are actually initialized with 0 (like I expect).
Here's a sample:

Default 0: 1875674416
Default 1: 32568
Default 2: 0
Default 3: 0
Default 4: 4199168
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So, is there a way I can get these integral types to initialize properly? Or do I have to manually initialize the array with 0 (somehow)?

Discussion (4)

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Ben Lovy

I'm actually not sure why your bool example works, maybe someone else can chime in. Your declaration is not an initialization. The std::array will perform default initialization, which crucially is not the same as value-initialization. For primitive types, each element is undefined, meaning it has garbage. It's different for std::string because that's a class.

To fix it, you can ask for value-initialization instead using std::array<int, 5>{};.

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Pierre Gradot

We can't answer without a minimal code that reproduces that the issue.

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Calin Baenen Author

Literally just do std::array<NumericDatatype, 5> arr; /* Code to print. */ and you'll get similar results.

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Pierre Gradot • Edited on

arr is not initialized. So its actual content is not specified.

The behavior you observe is normal: there are random values in the array.

Sometimes, random values look like something you expect (like : all bools are false) but this is pure coincidence.