Is it true that a default constructor is synthesized for every class that does not define one?

Question

If the class doesn\'t have the constructor, will the compiler make one default constructor for it ?

Programmers new to C++ often have two common misu

Answer 1

All the upvoted answers thus far seem to say approximately the same thing:

A default constructor is synthesized for every class that does not have any user-defined constructor.

which is a modification of the statement in the question, which means

A default constructor is synthesized for every class that does not have a user-defined default constructor.

The difference is important, but the statement is still wrong.

A correct statement would be:

A default constructor is synthesized for every class that does not have any user-defined constructor and for which all sub-objects are default-constructible in the context of the class.

Here are some clear counter-examples to the first statement:

struct NoDefaultConstructor
{
     NoDefaultConstructor(int);
};

class Surprise1
{
     NoDefaultConstructor m;
} s1; // fails, no default constructor exists for Surprise1

class Surprise1 has no user-defined constructors, but no default constructor is synthesized.

It doesn't matter whether the subobject is a member or a base:

class Surprise2 : public NoDefaultConstructor
{
} s2; // fails, no default constructor exists for Surprise2

Even if all subobjects are default-constructible, the default constructor has to be accessible from the composite class:

class NonPublicConstructor
{
protected:
    NonPublicConstructor();
};

class Surprise3
{
    NonPublicConstructor m;
} s3; // fails, no default constructor exists for Surprise3