Andrew Sutton wrote:
It should not. Equality should mean equality. I don't see why this should be so distasteful.
First, unnecessarily requiring both sides to be of the same type causes problems with (for instance) signed char c1 = -1; unsigned char c2 = 0xFF; where c1 != c2, but they compare equal if either is converted to the type of the other. So this requirement leads to much extra complexity with common_type and so on. Second, "test" == std::string( "test" ) == "test" but "test" may well not compare equal to "test". Third, because of the preceding two points, std::find does not, in practice, require same type or equivalence. This leads people to use it with types of the form struct X { int key_; ... value_; bool operator==( int key ) const { return key_ == key; } }; Now... this is where you legitimately may claim that these people are wrong. But I don't see why we need to prohibit this code. It makes no intuitive sense to me for std::find( first, last, value ) to differ from std::find_if( first, last, _1 == value ).