What's the difference between emplace & insert in std::vector or in std::list

[d_stranz]

In both cases, the vector or list will be grown if necessary to accommodate the new entry. size() will increase by 1, but capacity() may not if the underlying implementation's default allocation size is already big enough to hold the extra element or if you called reserve() with a larger value than the size() before the new element was added.

v.emplace(v.end(), m); // "emplace() constructs a new instance in place" --- I have passed a already constructed element.

Ah, but here you are implicitly invoking the MyClass copy constructor because you are passing a MyClass instance as an argument:

MyClass::MyClass( const MyClass & myClass )

MyClass::MyClass( const MyClass & myClass )

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so you end up with two instances, "m" - the one you constructed manually, and a copy of "m", built in place in the vector and initialized with the contents of "m". Even though your class doesn't explicitly define a copy constructor, C++ implements one for you (which by default makes a bitwise copy of the contents of the instance it is initialized with). Likewise for operator=().

To explain in another way:

insert() requires an already-existing instance of the class, which it then makes a copy of into the vector (using MyClass:: operator=( const MyClass & ))
emplace() does not require an existing instance, but creates one for you and initializes it with the extra arguments you pass in (using MyClass:: MyClass( arguments ))

[rawfool]

Ok, now I got it.
After reading your answer, I now tried with explicit constructor and it reported error for passing int value at insert. Now it makes sense to me.

explicit MyClass(int i = 0) : d_val(i) {}
...
...
v.insert(v.end(), 23);    // compiler reports error

explicit MyClass(int i = 0) : d_val(i) {}
...
...
v.insert(v.end(), 23);    // compiler reports error

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Thanks a lot, d_stranz

[d_stranz]

Yes, I missed your first example of insert( v.end(), 23 ). That would invoke the default constructor to create a temporary instance initialized to 23, which would then be copied into the vector.

Glad to help.