Uniformity of the Struct element without overloading the == operator in C ++

Question

Is it possible to define some kind of template that can create a common comparable operator for structures?

For example, is something like this possible?

struct A { int one; int two; int three; }; bool AreEqual() { A a {1,2,3}; A b {1,2,3}; return ComparableStruct<A>(a) == ComparableStruct<A>(b); } 

All this makes the field a field mapping of structures. You can assume that all fields have basic types or have an overloaded operator ==.

I have many such structures, and this will save me a lot of time if I can just put it in a template or something for comparison, and not define the == operator for each individual structure. Thanks!

Update

This seems to be impossible with C ++. I wonder why this is rejected from C ++ suggestions, if anyone has a reason, let us know!

For a solution that works with basic types, only see the solution from R Sahu.

Answer 1

Is it possible to define some kind of template that can create a common comparable operator for structures?

If the struct is indented, you can use:

 template <typename T> struct ComparableStruct { ComparableStruct(T const& a) : a_(a) {} bool operator==(ComparableStruct const& rhs) const { return (std::memcmp(reinterpret_cast<char const*>(&a_), reinterpret_cast<char const*>(&rhs.a_), sizeof(T)) == 0); } T const& a_; }; 

Even better, you can use a function template.

 template <typename T> bool AreEqual(T cost& a, T const& b) { return (std::memcmp(reinterpret_cast<char const*>(&a), reinterpret_cast<char const*>(&b), sizeof(T)) == 0); } 

If a struct has any addition, there is no guarantee that using std::memcmp will work to compare two objects.

Answer 2

Take a look at https://github.com/apolukhin/magic_get . This library can automatically generate comparison operators for some fairly simple structures.

 #include <iostream> #include <boost/pfr/flat/global_ops.hpp> struct S { char c; int i; double d; }; int main() { S s1{'a', 1, 100.500}; S s2 = s1; S s3{'a', 2, 100.500}; std::cout << "s1 " << ((s1 == s2) ? "==" : "!=") << " s2\n"; std::cout << "s1 " << ((s1 == s3) ? "==" : "!=") << " s3\n"; } // Produces // s1 == s2 // s1 != s3 

Answer 3

What you are asking to do is go through different structures and compare the participants, this is my understanding.

Structure iteration

It seems like this cannot be done with standard C ++, but this thread gives some ideas on which libraries to use.

It is not clear from your question whether all structures have the same format or not, and I assume that they do not.