How do you 'realloc' in C++?

Use ::std::vector!

Type* t = (Type*)malloc(sizeof(Type)*n) 
memset(t, 0, sizeof(Type)*m)

becomes

::std::vector<Type> t(n, 0);

Then

t = (Type*)realloc(t, sizeof(Type) * n2);

becomes

t.resize(n2);

If you want to pass pointer into function, instead of

Foo(t)

use

Foo(&t[0])

It is absolutely correct C++ code, because vector is a smart C-array.

The right option is probably to use a container that does the work for you, like std::vector.

new and delete cannot resize, because they allocate just enough memory to hold an object of the given type. The size of a given type will never change. There are new[] and delete[] but there's hardly ever a reason to use them.

What realloc does in C is likely to be just a malloc, memcpy and free, anyway, although memory managers are allowed to do something clever if there is enough contiguous free memory available.