CS253: Software Development with C++
Spring 2018
Dynamic Memory Allocation
In C, we used functions to allocate & free memory:
They still work, but don’t use them. They’re not type-safe, and they don’t call ctors & dtors (constructors and destructors).
In C++, we use keywords to allocate & free memory:
new
delete
new []
delete []
You have to delete the memory that you allocate.
Don’t delete it more than once!
int *p = new int; *p = 42; cout << p << ' ' << *p << '\n'; delete p;
0x242e2b0 42
int *a = new int[10]; for (int i=0; i<10; i++) a[i] = i*11; for (int i=0; i<10; i++) cout << a[i] << ","; delete[] a; // Note the []
0,11,22,33,44,55,66,77,88,99,
new with delete, and new [] with delete []
new, you must call delete.
new [], you must call delete [].
If you mix them up, the system might catch your error, or it might not.
Java programmers, remember that objects do not have to be dynamically allocated. You can, but you don’t have to.
string s = new string;
c.cc:1: error: conversion from 'std::__cxx11::string*' {aka
'std::__cxx11::basic_string<char>*'} to non-scalar type
'std::__cxx11::string' {aka 'std::__cxx11::basic_string<char>'} requested
Instead, just declare the string:
string s = "Hi there\n"; cout << s;
Hi there
Sure, the string allocates dynamic memory, internally, but that’s none of your business.
In general, use standard containers such as string, vector, or list when you can. They handle the dynamic memory allocation, so you don’t have to.
If that’s not possible, then consider unique_ptr and shared_ptr.
