Есть ли разница между MyClass *c = new MyClass() и MyClass *c = new MyClasss;

FarStar, Все равно, есть user-defined конструктор, потому всегда будет вызван он. А вот какова будет инициализация в конструкторе - дело того кто его пишет.

быдлером

Что за STD?

Но когда проект содержит несколько сотен тысяч строк

если нет юзер конструктора, тогда что?

A new-expression that creates an object of type T initializes that object as follows:
— If the new-initializer is omitted:
— If T is a (possibly cv-qualified) non-POD class type (or array thereof), the object is default-
initialized (8.5). If T is a const-qualified type, the underlying class type shall have a user-declared
default constructor.
— Otherwise, the object created has indeterminate value. If T is a const-qualified type, or a (possibly
cv-qualified) POD class type (or array thereof) containing (directly or indirectly) a member of
const-qualified type, the program is ill-formed;
— If the new-initializer is of the form (), the item is value-initialized (8.5);
— If the new-initializer is of the form (expression-list) and T is a class type, the appropriate constructor is
called, using expression-list as the arguments (8.5);
— If the new-initializer is of the form (expression-list) and T is an arithmetic, enumeration, pointer, or
pointer-to-member type and expression-list comprises exactly one expression, then the object is initial-
ized to the (possibly converted) value of the expression (8.5);
— Otherwise the new-expression is ill-formed.

To zero-initialize an object of type T means:
— if T is a scalar type (3.9), the object is set to the value of 0 (zero) converted to T;
— if T is a non-union class type, each nonstatic data member and each base-class subobject is zero-
initialized;
— if T is a union type, the object’s first named data member89) is zero-initialized;
— if T is an array type, each element is zero-initialized;
— if T is a reference type, no initialization is performed.
To default-initialize an object of type T means:
— if T is a non-POD class type (clause 9), the default constructor for T is called (and the initialization is
ill-formed if T has no accessible default constructor);
— if T is an array type, each element is default-initialized;
— otherwise, the object is zero-initialized.
To value-initialize an object of type T means:
— if T is a class type (clause 9) with a user-declared constructor (12.1), then the default constructor for T is
called (and the initialization is ill-formed if T has no accessible default constructor);
— if T is a non-union class type without a user-declared constructor, then every non-static data member
and base-class component of T is value-initialized;
— if T is an array type, then each element is value-initialized;
— otherwise, the object is zero-initialized

Scalar types, POD-struct types,
POD-union types (clause 9), arrays of such types and cv-qualified versions of these types (3.9.3) are collec-
tively called POD types.

A POD-struct is an aggregate class that has no non-static data members of type non-POD-struct,
non-POD-union (or array of such types) or reference, and has no user-defined copy assignment operator
and no user-defined destructor.

назовите на чем вы сейчас пишите, я тоже хочу назвать эту среду гамном имеющей недостатки

ну типа std::и_все_что_тут_пишется

Я пожалуй, слегка был не прав, приведя цитаты из стандарта С++11. Тут слегка изменились правила. Вот из 03.

и что будет происходить если конструктор без параметров был запрещен в определении типа?

похаить VS2010

То массив создать не получится. При создании массива вызывается конструктор по умолчанию.