CODEWITHSUNDEEP
c++c++11
Nater
Nater

Reputation: 165

C ++ associated container custom type is initialized in the class

Two initialization codes for association container objects of custom type comparison first , not in class:

class A
{
public:
   int a;
};
bool compare(const A &a1 , const A &a2)
{ return a1.a > a2.a; }
multiset<A , decltype(compare)> ml(compare);

this code is safety but () initialization statement cannot be used in the class 

class z
{
public:
    bool compare(const A &a1 , const A &a2)
    { return a1.a > a2.a; }
    std::multiset<A , decltype(compare)> ml(compare);
};

this code use () to initialize the container object will wrong, only can use {} 

std::multiset<A , decltype(compare)> ml{compare};

this code is safety , why?

Upvotes: 0

Views: 30

Answers (1)

ypnos
ypnos

Reputation: 52317

This is not particular to your compare. While C++11 allows to initialize class members within the definition, you do that either with {} (brace initializer) or with assignment (equals initializer).

struct A {
    int a = 42; // works
    int a{42}; // works
    int a(42); // unsupported syntax
};

In your example, this would work:

std::multiset<A, decltype(compare)> ml = std::multiset<A, decltype(compare)>(compare);

Upvotes: 1

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