26 lines
946 B
Markdown
26 lines
946 B
Markdown
# Resizing
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We said that `Vec` is a "growable" vector type, but what does that mean?
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What happens if you try to insert an element into a `Vec` that's already at maximum capacity?
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```rust
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let mut numbers = Vec::with_capacity(3);
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numbers.push(1);
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numbers.push(2);
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numbers.push(3); // Max capacity reached
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numbers.push(4); // What happens here?
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```
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The `Vec` will **resize** itself.
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It will ask the allocator for a new (larger) chunk of heap memory, copy the elements over, and deallocate the old memory.
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This operation can be expensive, as it involves a new memory allocation and copying all existing elements.
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## `Vec::with_capacity`
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If you have a rough idea of how many elements you'll store in a `Vec`, you can use the `Vec::with_capacity`
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method to pre-allocate enough memory upfront.
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This can avoid a new allocation when the `Vec` grows, but it may waste memory if you overestimate actual usage.
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Evaluate on a case-by-case basis.
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