On Wed, 2 Sep 2020 at 21:46, Steve Hearnden via Std-Proposals
<std-proposals_at_[hidden]> wrote:
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> I was looking at a performance issue, with some code and found that the cost of creating the dynamic heap memory was the significant cost.
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> It would have been handy to have a class which implemented something like std::vector, but with a *small* buffer of values to use as an alternative piece of memory when the vector size was below a threshold.
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> Unfortunately, an array of the constructed type, would cause the constructor to be called for each array element, and for complex types, could undo the benefit of the pre-bought memory.
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> My proposal would be for classes (or structures) to have the ability for a member to be declared with an attribute
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> [[unconstructed]]
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> which would cause none of the constructors to actually construct the object.
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> The class would not naturally also call destructors.
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> e.g.
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> template< class T, size_t size>
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> class temp_vector
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> {
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> [[unconstructed]] T _V[size];
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> size_t m_constructed = 0;
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> public:
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> ~temp_vector()
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> {
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> ///// Destroy the array manually.
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> for( size_t i = 0; i < m_constructed; i++ ){
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> _V[i].~T();
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> }

Why don't you just use std::optional for the storage?