How do you initialise a dynamic array in C++?

char* c = new char[length]();

Two ways:

char *c = new char[length];
std::fill(c, c + length, INITIAL_VALUE);
// just this once, since it's char, you could use memset

Or:

std::vector<char> c(length, INITIAL_VALUE);

In my second way, the default second parameter is 0 already, so in your case it's unnecessary:

std::vector<char> c(length);

[Edit: go vote for Fred's answer, char* c = new char[length]();]

Maybe use std::fill_n()?

char* c = new char[length];
std::fill_n(c,length,0);

The array form of new-expression accepts only one form of initializer: an empty (). This, BTW, has the same effect as the empty {} in your non-dynamic initialization.

The above applies to pre-C++11 language. Starting from C++11 one can use uniform initialization syntax with array new-expressions

char* c = new char[length]{};
char* d = new char[length]{ 'a', 'b', 'c' };

C++ has no specific feature to do that. However, if you use a std::vector instead of an array (as you probably should do) then you can specify a value to initialise the vector with.

std::vector <char> v( 100, 42 );

creates a vector of size 100 with all values initialised to 42.

You can't do it in one line easily. You can do:

char* c = new char[length];
memset(c, 0, length);

Or, you can overload the new operator:

void *operator new(size_t size, bool nullify)
{
    void *buf = malloc(size);

    if (!buf) {
             // Handle this
    }

    memset(buf, '\0', size);

    return buf;
}

Then you will be able to do:

char* c = new(true) char[length];

while

char* c = new char[length];

will maintain the old behavior. (Note, if you want all news to zero out what they create, you can do it by using the same above but taking out the bool nullify part).

Do note that if you choose the second path you should overload the standard new operator (the one without the bool) and the delete operator too. This is because here you're using malloc(), and the standard says that malloc() + delete operations are undefined. So you have to overload delete to use free(), and the normal new to use malloc().

In practice though all implementations use malloc()/free() themselves internally, so even if you don't do it most likely you won't run into any problems (except language lawyers yelling at you)

No internal means, AFAIK. Use this: memset(c, 0, length);

and the implicit comment by many posters => Dont use arrays, use vectors. All of the benefits of arrays with none of the downsides. PLus you get lots of other goodies

If you dont know STL, read Josuttis The C++ standard library and meyers effective STL

you have to initialize it "by hand" :

char* c = new char[length];
for(int i = 0;i<length;i++)
    c[i]='\0';