Re: Inserting objects into a std::map?
saneman wrote:
I am reading section 23 in the C++ standard and cannot seem to find
where it says that the key_type must be defined for '<' when inserting
into std::map.
It is described in The C++ Programming Language 3th ed so I assume I
just can't find it.
Another thing. When I make:
std::map<Bob, int> m;
And my Bob class does not define '<' operator why does the compiler not
complain in the above declaration of 'm'?
It's called SFNIAE (Substitution Failure Is Not An Error). Until the
problematic code is instantiated (by the insert call below), there isn't
a problem.
Note that you don't have to define '<', you *could* specialize
std::less<>, or you could create a separate functor/function for
comparison, and use that as the third template argument to std::map.
Its first when I insert that the compiler complains about the missing
operator in Bob:
Bob b1(22);
std::pair<Bob, int> p_bob2int;
p_bob2int.first = b1; // key.
p_bob2int.second = 23; // mapped type.
m.insert(p_bob2int); // DOES NOT WORK UNLESS Bob IMPLEMENTS <
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