Should we need to provide a default constructor in c++ if we use a parameter constructor?

Question

Should we need to provide a default constructor in c++ if we use a parameter constructor? my code is like this.

ReadConfigParams::ReadConfigParams(char *file)

@user3159253: `std::vector` does not require its elements to be default constructible. — Benjamin Lindley, Apr 28 '14 at 04:57
@Ben, perhaps the constructor intends to open a file, and `ifstream::open` expects `char *` ..:) — M.M, Apr 28 '14 at 05:30
@Matt McNabb This has been fixed in c++11, and you can always use `std::string::c_str()` to convert to a zero-terminated `const char *`. (Also, use const correctness) ;) — tillaert, Apr 28 '14 at 05:49

score 1 · Answer 1 · answered Apr 28 '14 at 05:30

It depends on how you want a client of ReadConfigParams class should instantiate an object of this class.

If the class ReadConfigParams always requires a file name (as in most of the cases), you should NOT define a default constructor because it doesn't make sense to instantiate ReadConfigParams without an associated file. This will effectively prevent a client from creating an object of this class without a given file name.

But if the class ReadConfigParams can provide some default configuration values then you SHOULD define a default constructor. This will enable a client of this class to instantiate and hence access the default values without a file reading operation.

score 1 · Answer 2 · edited May 23 '17 at 10:28

Usually, if you don't declare a default constructor for a class, the compiler supplies one for you.

However, note that, the default constructor will only be created by the compiler if you provide no constructor whatsoever. For your example, you have provided one constructor that takes one argument, the compiler will then NOT create the default constructor for you.

Example-1:

class A
{
};

int main() 
{
    A a; // OK, as the compiler creates one for you
}

Example-2:

class A
{
public:
    A(int){}
};

int main() 
{
    A a; // NOT OK, error: no appropriate default constructor available!
}

Example-3:

class A
{
public:
    A(){}
    A(int){}
};

int main() 
{
    A a; // OK, as you explicitly provide one default constructor
}

Also, according to "C++11 – ISO-IEC 14882-2011" §8.5/6:

If a program calls for the default initialization of an object of a const-qualified type T, T shall be a class type with a user-provided default constructor.

You can check out this thread for more info.

score 0 · Answer 3 · answered Apr 28 '14 at 06:02

Question Should we need to provide a default constructor in c++ if we use a parameter constructor?

Answer It depends.

If you wish to create an array of objects, you must provide a default constructor.

ReadConfigParams array[10];

won't work if you have a don't have a default constructor. Similarly, you can't allocate an array of object using operator new.

ReadConfigParams* arrayPtr = new ReadConfigParams[10];

won't work either.

If you don't have any need for creating arrays, in stack or from heap, not providing a default constructor is fine.

Should we need to provide a default constructor in c++ if we use a parameter constructor?

3 Answers3