constexprStd

This library provides some part of the STL as constexpr version. The implementation may not be as efficient as your standard STL, but can be used at compile time.

As it is an implementation it contains no API documentation, for that I recommend cppreference.com. In addition to supply constexpr versions of functions which are not declared constexpr by the standard, it also contains some fixes. For example std::advance should be constexpr as of C++17, but the GCC implementation of it has a bug in 7.1 and 7.2, see Bug 81912.

Usage

As this is a constexpr library just clone it into your project directory and add include to your includepath. Projects based on Qt can include(constexprStd/constexprStd.pri) in their project file.

And then you use the library just like the STL.
Instead of #include <algorithm> you write #include <constexprStd/algorithm> and the content is in the namespace constexprStd:: instead of std::.

Noteable differences to std::

General

I've added conditional noexcept statements to nearly all (member) functions. So what can be noexcept should also be marked as such. I've only omited the statements when a throw is actually used in its body.

Allocator

Since we can neither use placement new, nor reinterpret_cast at compile time, the allocator does not fulfill the allocator concept and can probably not be used for std:: containers.

It lacks the copy/move construction and assignment. The equality operator tests if it is the same object. And rebinding is not possible.

When allocating multiple elements you can not easyly construct them in constexpr context.

Allocator<T, N> a;
T *singleElement = a.allocate(1);
T *multipleElements = a.allocate(3);

a.construct(singleElement, ...); //This is fine
a.construct(multipleElements, ...); //This also, initializes the first element
a.constrcut(&multipleElements[0], ...); //Works also
a.constrcut(&multipleElements[1], ...); //Does not work in a constexpr context, does work at runtime

//What you have to do is:
T *multiSecondElement = a.getPointer(multipleElements, 1); //This is a operation in O(N), so use sparely
//                                   ^^                ^
//                            base pointer of array    offset to the pointer, here multipleElements[1]
a.construct(multiSecondElement, ...);

//But you can provide a hint, so it is faster
T *multiThirdElement = a.getPointer(multipleElements, 2, static_cast<std::size_t>(constexprStd::distance(singleElement, multipleElements)));
//                                  ^^^^^^^^^^^^^^^^^^^  ^^                              ^^^^^^^^^^^^^^^^
//                                  both as before       cast if you use -Wconversion    the actual hint, the difference from the first element to the base pointer
a.construct(multiThirdElement, ...);

Even if you provide a hint which is after the actual base pointer the algorithm is correct and complete, but the runtime is as bad as it can get. If you do not have access to the very first element, you can use what ever you have, that is before our base pointer, everything helps.

constexprStd::variant

Incompatibilites to std::variant

Using types with a non trivial destructor is not possible in constexpr context, so they can't be used in constexrStd::variant either. But all other types can be used, even emplace and the assignment operator, which are not constexpr in their std:: incarnation. This comes at the cost, that the implementation does not meet all requirements. When emplacing a new type, the value is not direct initialized, but move assigned to the storage where another type was before. This means that some types are not save to be used in constexprStd::variant. If you want to use such a type and need direct initialization, add a non trivial destructor to it and the variant behaves like std::variant.

Additions

• It is possible to (un-)equal compare different types of variant. They only compare as equal, iff both hold the same type and the equal comparison of this type returns true.
• It is possible to assign the content of diffrent types of variant. But only if the currently hold alternative of the right hand side is also an alternative of the left hand side, or the right hand side is valueless. In any other case a constexprStd::BadVariantAssignment exception is thrown.

constexprStd::pair

The default constructor is never explicit.

constexprStd::set

Because it is based on our Allocator it's not fully compatible with std::set, we do not provide extract() and the corresponding insert() functions. And because we can not call a non trivial destructor in constexpr constexprStd::set does NOT delete the content of the nodes in its destructor. In constexpr mode this is not necessary, because only types with a non trivial destructor could leak memory, but if you intend to use the set in run time (which is advised against) you have to manually call clear or use the constexprStd::setDestroy wrapper.

Note that emplace() and emplace_hint() are not constexpr, because without copying or moving we could not provide exception safety.

It the provided constexprStd::allocator runs out of memory it falls back to std::allocator. This results in a compile time error, when happening in constexpr mode, in this case you have to increase the preallocated memory.

Algorithms

• constexprStd::stable_partition never runs in O(n) swaps, because we can not allocate extra memory.

Implementation

Algorihms library

The execution policies and the overloads for the algorithms are not implemented, because as far as I know, there is no chance of performing paralell computations in constexpr. But we offer some convenience overloads, e.g. instead of copy(c.begin(), c.end(), dest) you can call copy(c, dest).

Non-modifying sequence operations

• all_of
• any_of
• none_of
• for_each
• for_each_n
• count
• count_if
• mismatch
• equal
• find
• find_if
• find_if_not
• find_end
• find_first_of
• adjacent_find
• search
• search_n

Modifying sequence operations

• copy
• copy_if
• copy_n
• copy_backward
• move
• move_backward
• fill
• fill_n
• transform
• generate
• generate_n
• remove
• remove_if
• remove_copy
• remove_copy_if
• replace
• replace_if
• replace_copy
• replace_copy_if
• swap_ranges
• iter_swap
• reverse
• reverse_copy
• rotate
• rotate_copy
• shuffle
• sample
• unique
• unique_copy

Partitioning operations

• is_partitioned
• partition
• partition_copy
• stable_partition
• partition_point

Sorting operations

• is_sorted
• is_sorted_until
• sort
• partial_sort
• partial_sort_copy
• stable_sort
• nth_element

Binary search operations (on sorted ranges)

• lower_bound
• upper_bound
• binary_search
• equal_range

Set operations (on sorted ranges)

• merge
• inplace_merge
• includes
• set_difference
• set_intersection
• set_symmetric_difference
• set_union

Heap operations

• is_heap
• is_heap_until
• make_heap
• push_heap
• pop_heap
• sort_heap

Minimum/maximum operations

• max
• max_element
• min
• min_element
• minmax
• minmax_element
• clamp
• lexicographical_compare

Permutations

• is_permutation
• next_permutation
• prev_permutation

Numeric operations

• iota
• accumulate
• inner_product
• adjacent_difference
• partial_sum
• reduce
• exclusive_scan
• inclusive_scan
• transform_reduce
• transform_exclusive_scan
• transform_inclusive_scan

Operations on uninitialized memory

• uninitialized_copy
• uninitialized_copy_n
• uninitialized_fill
• uninitialized_fill_n
• uninitialized_move
• uninitialized_move_n
• uninitialized_default_construct
• uninitialized_default_construct_n
• uninitialized_value_construct
• uninitialized_value_construct_n
• destroy_at
• destroy
• destroy_n

Function objects

Polymorphic function wrappers

• function
• mem_fn
• bad_function_call: If used, use std::.
• invoke

Negators

• not_fn

Searchers

• default_searcher
• boyer_moore_searcher
• boyer_moore_horspool_searcher

Iterator library

Iterator primitives

These can be used directly from the std:: namespace.

Iterator adaptors

• reverse_iterator: This can be used directly from the std:: namespace.
• make_reverse_iterator: This can be used directly from the std:: namespace.
• move_iterator
• make_move_iterator
• back_insert_iterator
• back_inserter
• front_insert_iterator
• front_inserter
• insert_iterator
• inserter

Stream iterators

Has to be checked if there are actually constexpr streams.

Iterator operations

Are actually defined as constexpr, but as stated earlier there are bugs in the GCC implementation, so we provide them too.

• advance
• distance
• next
• prev

Range access & Container access

These can be used directly from the std:: namespace.

"Dynamic" memory management

Allocators

• allocator

Utility library

• bitset

optional, variant and any

• optional
• variant
• any
• in_place*: These can be used directly from the std:: namespace.

Pairs and tuples

• pair
• tuple
• apply: This can be used directly from the std:: namespace.
• make_from_tuple: This can be used directly from the std:: namespace.
• piecewise_construct_t: This can be used directly from the std:: namespace.
• piecewise_construct: This can be used directly from the std:: namespace.
• integer_sequence: This can be used directly from the std:: namespace.

Swap, forward and move

• swap
• exchange

The remainder is constexpr.

Type operations

These can be used directly from the std:: namespace.

Relational operators

• operator!=
• operator>
• operator<=
• operator>=

Elementary string conversions

• to_chars
• from_chars
• chars_format: This can be used directly from the std:: namespace.

Hash support

• hash

Containers Library

• set