Are you looking to understand how to compare two vectors in C++ efficiently? COMPARE.EDU.VN offers a comprehensive guide on comparing std::span objects, especially with C++20 features. This article will show you how to effectively use concepts, ranges, and the spaceship operator for accurate and optimized comparisons.
1. Introduction: Comparing Vectors in C++
Comparing vectors is a fundamental operation in C++. The std::span in C++20 provides a non-owning view over a contiguous sequence of objects, allowing for efficient data manipulation. Understanding how to compare std::span objects and their underlying data is essential. While earlier designs of std::span included deep comparison operators, these were later removed. This article explores how to implement these comparison operators effectively, showcasing C++20 features. At COMPARE.EDU.VN, we aim to give you the best information so that you can confidently compare vectors.
2. Design Considerations for Span Comparison
Before implementing comparison operators for spans, you need to decide which types a std::span should be comparable with. Here are several potential types for comparison:
std::span<T, Extent>std::span<T, Extent>std::span<T, dynamic_extent>std::vector<T>std::list<T>std::vector<T>
For a clear comparison, std::span should work, as const-ness should not affect comparisons. It’s logical to compare std::span with std::vector because both represent contiguous ranges. However, comparing std::span with std::list or std::vector may not be directly appropriate, as span comparisons should ideally behave like memcmp, suitable for contiguous ranges of the same type. The best syntax should allow direct comparisons (e.g., s == v) for valid cases, while others can use std::ranges::equal. This approach differs from the original span proposal, which limited comparisons to std::span only.
3. Implementing Comparison Operators in C++17
How do you implement comparison operators in C++17, given the above design? Cross-type comparisons should be allowed in both directions to ensure consistency (i.e., if s == v compiles, v == s should also compile and produce the same result). Ignoring constraints for simplicity, start with the following templates:
namespace std {
template <typename T, size_t Extent = dynamic_extent>
class span { /* ... */ };
template <typename T, size_t Extent, typename R>
bool operator==(span<T, Extent> lhs, R const& rhs) {
// Avoid ADL shenanigans by using qualified call to std::equal
return std::equal(begin(lhs), end(lhs), begin(rhs), end(rhs));
}
template <typename T, size_t Extent, typename R>
bool operator==(R const& lhs, span<T, Extent> rhs) {
return rhs == lhs;
}
// and != overloads that just call ==
template <typename T, size_t Extent, typename R>
bool operator!=(span<T, Extent> lhs, R const& rhs) {
return !(lhs == rhs);
}
template <typename T, size_t Extent, typename R>
bool operator!=(R const& lhs, span<T, Extent> rhs) {
return !(rhs == lhs);
}
// ... and <, <=, >, >=
}The implementation for a specific comparison operator is straightforward, using standard algorithms. However, just adding these operators results in ambiguity when comparing two spans of the same type:
std::span<int> x = /* ... */;
std::span<int> y = /* ... */;
bool check = (x == y);This produces an error because there are two viable operator== candidates: one templated on the left argument and the other on the right, with neither being more specialized.
4. Resolving Ambiguity with Additional Overloads
One way to resolve the ambiguity is to add more overloads. Include a third candidate for operator== specifically for comparing two spans:
namespace std {
template <typename T, size_t Extent = dynamic_extent>
class span { /* ... */ };
template <typename T, size_t Extent, typename R>
bool operator==(span<T, Extent> lhs, R const& rhs);
template <typename T, size_t Extent, typename R>
bool operator==(R const& lhs, span<T, Extent> rhs);
template <typename T, size_t E1, size_t E2>
bool operator==(span<T, E1>, span<T, E2>);
}Here, the extents can differ, but the underlying types must be the same. By comparing only contiguous elements, both generic range comparison operators can be directed to the span-specific one. A complete solution for operator== with proper constraints might look like this:
#define REQUIRES(...) std::enable_if_t<(__VA_ARGS__), int> = 0
// Check if we can compare two Ts
template <typename T>
inline constexpr bool equality_comparable_v =
std::is_invocable_v<std::equal_to, T const&, T const&> &&
std::is_invocable_v<std::not_equal_to, T const&, T const&>;
namespace std {
template <typename T, size_t Extent = dynamic_extent>
class span { /* ... */ };
template <typename T, size_t E1, size_t E2, REQUIRES(equality_comparable_v<T>)>
bool operator==(span<T, E1> lhs, span<T, E2> rhs) {
// Could even check if E1/E2 are different fixed extents, return false
return std::equal(lhs.begin(), lhs.end(), rhs.begin(), rhs.end());
}
template <typename T, size_t E, typename R,
REQUIRES(std::is_constructible_v<span<T>, R const&> && equality_comparable_v<T>)>
bool operator==(span<T, E> lhs, R const& rhs) {
return lhs == std::span<T>(rhs);
}
template <typename T, size_t E, typename R,
REQUIRES(std::is_constructible_v<span<T>, R const&> && equality_comparable_v<T>)>
bool operator==(R const& lhs, span<T, E> rhs) {
return rhs == std::span<T>(lhs);
}
}This resolves the earlier comparison problem. However, this must be repeated for every comparison operator, totaling 18 operators, most of which simply forward to the core implementations. This method allows comparing span to span but fails to compare span to span or span to vector because span is not constructible from vector<int> const&.
5. Introducing the sameish Constraint
To fix this, introduce a sameish constraint to ensure types are the same after removing const-ness.
#define REQUIRES(...) std::enable_if_t<(__VA_ARGS__), int> = 0
// Check if we can compare two Ts
template <typename T>
inline constexpr bool equality_comparable_v =
std::is_invocable_v<std::equal_to, T const&, T const&> &&
std::is_invocable_v<std::not_equal_to, T const&, T const&>;
template <typename T, typename U>
inline constexpr bool sameish = std::is_same_v<std::remove_cv_t<T>, std::remove_cv_t<U>>;
namespace std {
template <typename T, size_t Extent = dynamic_extent>
class span { /* ... */ };
template <typename T, typename U, size_t E1, size_t E2,
REQUIRES(equality_comparable_v<T> && sameish<T, U>)>
bool operator==(span<T, E1> lhs, span<U, E2> rhs) {
return std::equal(lhs.begin(), lhs.end(), rhs.begin(), rhs.end());
}
template <typename T, size_t E, typename R,
REQUIRES(std::is_constructible_v<span const, R const&> && equality_comparable_v<T>)>
bool operator==(span<T, E> lhs, R const& rhs) {
return lhs == std::span<T const>(rhs);
}
template <typename T, size_t E, typename R,
REQUIRES(std::is_constructible_v<span const, R const&> && equality_comparable_v<T>)>
bool operator==(R const& lhs, span<T, E> rhs) {
return rhs == std::span<T const>(lhs);
}
}This complete solution requires checking that T == T is a valid expression and convertible to bool. Although it satisfies the set requirements, it necessitates writing 18 operators and careful constraint management.
6. C++17 Implementation with Hidden Friends
Another technique is using hidden friends to implement comparison operators.
namespace std {
template <typename T, size_t Extent = dynamic_extent>
class span {
// ...
template <typename U = T, REQUIRES(equality_comparable_v<U>)>
friend bool operator==(span lhs, span rhs);
};
}This approach does not directly enable comparing a vector<int> const to a span. If the hidden friend takes two span objects, it leads to redefinition issues between span and span defining the same operators. Therefore, hidden friends do not provide a straightforward solution in this context.
7. Implementing Comparison Operators in C++20
C++20 offers significant improvements for implementing comparison operators through:
- Concepts
- Ranges
operator<=>(Spaceship Operator)
The initial design dictates that span should be comparable to any contiguous range of the sameish type, provided that type is comparable. Turn this into a concept:
// A contiguous range whose value type and T
// are the sameish type
// (range_value_t shouldn't be cv-qualified)
template <typename R, typename T>
concept const_contiguous_range_of =
std::ranges::contiguous_range<R const> &&
std::same_as<std::remove_cvref_t<T>, std::ranges::range_value_t<R const>>;Translate these requirements into code:
namespace std {
template <typename T, size_t Extent = dynamic_extent>
class span { /* ... */ };
template <typename T, size_t E, typename R,
REQUIRES(const_contiguous_range_of<R, T> && equality_comparable_v<T>)>
bool operator==(span<T, E> lhs, R const& rhs) {
return std::ranges::equal(lhs, rhs);
}
template <typename T, size_t E, typename R,
REQUIRES(const_contiguous_range_of<R, T> && std::three_way_comparable<T>)>
auto operator<=>(span<T, E> lhs, R const& rhs) {
return std::lexicographical_compare_three_way(
lhs.begin(), lhs.end(), std::ranges::begin(rhs), std::ranges::end(rhs));
}
}This complete solution involves just two operators with straightforward constraints. If types do not provide <=>, use synth-three-way and constrain based on that. These can be member function templates, though they are made non-member function templates here to add them outside the standard library. C++20 comparison operators enable reversed and synthesized comparisons, reducing the need for 18 operators.
8. Comparison Examples Using C++20
Here are several examples to illustrate how overload resolution works with C++20 comparison operators.
8.1. Comparing span to span with ==
Two candidates exist:
operator==(span<T>, R const&)withT=intandR=span.operator==(R const&, span<T>)withT=intandR=span(reversed).
Both are exact matches, but the non-reversed candidate is preferred, ensuring unambiguous and correct implementation.
8.2. Comparing span to span with ==
Again, two candidates exist:
operator==(span<T>, R const&)withT=intandR=span.operator==(R const&, span<T>)withT=int constandR=span.
Both candidates are exact matches, but the non-reversed candidate is preferred, resulting in unambiguous comparison.
8.3. Comparing span to span with !=
Since there is no operator!= candidate, this operates similarly to ==, but evaluates !(x == y). This leverages x == y to provide x != y.
8.4. Comparing vector to span with ==
The normal operator is not a candidate, as vector is not a span, but the reversed operator deduces R=vector and T=int, making it a viable candidate.
8.5. Comparing span to list with ==
Only the normal candidate is potentially viable, but R=list fails to satisfy the contiguous range constraint, resulting in no viable candidates.
8.6. Comparing span to vector with ==
The normal candidate is potentially viable, and R=vector is a contiguous range. However, int and long are not sameish, resulting in no viable candidates.
8.7. Comparing vector to span with <
No operator< candidates exist, but an operator<=> candidate exists with reversed parameters, deducing R=vector and T=int and satisfying all constraints. The code v < s is rewritten to 0 < operator<=>(s, v).
9. Conclusion: Streamlining Vector Comparisons in C++20
Implementing span‘s comparison operators in C++20 is remarkably easier than in C++17, thanks to concepts, ranges, and the spaceship operator. Although std does not provide these operators, external libraries like span_ext implement all comparison operators for std::span in C++20.
While libc++ may lack full C++20 library functionality, libraries like range-v3 can fill the gaps. Despite the missing comparisons for std::span, C++20 offers significant advantages. At COMPARE.EDU.VN, we strive to give you the best comparison information so that you can make the right choice.
10. Call to Action
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11. FAQ
Q1: Can I compare std::span with std::vector directly in C++20?
Yes, with the correct implementation using concepts, ranges, and the spaceship operator, you can directly compare std::span with std::vector.
Q2: Why were the comparison operators removed from the original std::span design?
The comparison operators were removed to simplify the standard and avoid potential issues with implicit conversions and unexpected behavior.
Q3: What is the sameish constraint used for?
The sameish constraint ensures that two types are the same after removing const-ness, allowing for more flexible comparisons between const and non-const types.
Q4: How does C++20 simplify comparison operator implementation?
C++20 simplifies comparison operator implementation by allowing reversed and synthesized comparisons through the spaceship operator and concepts for clearer constraint management.
Q5: What is the role of the spaceship operator (<=>) in C++20 comparisons?
The spaceship operator allows for the generation of all six comparison operators (<, <=, ==, !=, >, >=) from a single function, simplifying the code and reducing redundancy.
Q6: Can I use hidden friends to implement comparison operators for std::span?
While hidden friends can be used, they do not directly solve the problem of comparing std::span with other container types like std::vector.
Q7: What is std::ranges::equal used for in the comparison?
std::ranges::equal is used to compare the elements within the std::span and the other range, ensuring that the comparison is done element-wise.
Q8: How do I handle types that do not provide the <=> operator in C++20?
For types that do not provide the <=> operator, you can use synth-three-way and constrain your comparison operators based on that.
Q9: What are the advantages of using concepts in C++20 for comparisons?
Concepts allow you to define constraints on template parameters, ensuring that the types being compared meet certain requirements, such as being equality-comparable or contiguous ranges.
Q10: Where can I find an implementation of comparison operators for std::span in C++20?
You can find an implementation in the span_ext repository, which provides comparison operators for std::span in C++20.
12. Contact Information
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This guide equips you with the knowledge to compare vectors in C++ effectively. Whether you’re dealing with std::span or other containers, understanding these comparison techniques is essential for writing robust and efficient code. Remember to visit compare.edu.vn for more in-depth comparisons and to stay ahead in your C++ endeavors.
