內存管理運算符 new、new[]、delete 和 delete[] 也可以進行重載，其重載形式既可以是類的成員函數，也可以是全局函數。一般情況下，內建的內存管理運算符就夠用了，只有在需要自己管理內存時才會重載。
以成員函數的形式重載 new 運算符：
void * className::operator new( size_t size ){ //TODO: }
以全局函數的形式重載 new 運算符：
void * operator new( size_t size ){ //TODO: }
兩種重載形式的返回值相同，都是void *類型，並且都有一個參數，為size_t類型。
在重載 new 或 new[] 時，無論是作為成員函數還是作為全局函數，它的第一個參數必須是 size_t 類型。size_t 表示的是要分配空間的大小，對於 new[] 的重載函數而言，size_t 則表示所需要分配的所有空間的總和。
重載的new必須有一個size_t參數。這個參數由編譯器產生並傳遞給我們，它是要分配內存的對象的長度。必須返回一個指向等於這個長度（或者大於這個長度）的對象的指針。如果沒有找到存儲單元（在這種情況下，構造函數不被調用），則返回一個0。然后如果找不到存儲單元，不能僅僅返回0，我們還應該調用new-handler或產生一個異常信息之類的事，告訴這里出現了問題。
我們首先需要明白的一點就是：operator new()的返回值是一個void*，而不是指向任何特定類型的指針。所做的是分配內存，而不是完成一個對象建立--直到構造函數調用了才完成對象的創建，它是編譯器確保做的動作，不在我們的控制范圍之內了，所以我們就沒有必要考慮。
size_t 在頭文件 <cstdio> 中被定義為typedef unsigned int size_t;，也就是無符號整型。
當然，重載函數也可以有其他參數，但都必須有默認值，並且第一個參數的類型必須是 size_t。
同樣的，delete 運算符也有兩種重載形式。以類的成員函數的形式進行重載：
void className::operator delete( void *ptr){ //TODO: }
以全局函數的形式進行重載：
void operator delete( void *ptr){ //TODO: }
兩種重載形式的返回值都是 void 類型，並且都必須有一個 void 類型的指針作為參數，該指針指向需要釋放的內存空間。
當我們以類的成員函數的形式重載了new 和 delete 操作符，其使用方法如下：
如果類中沒有定義 new 和 delete 的重載函數，那么會自動調用內建的 new 和 delete 運算符。
重載全局new和delete
#include <QCoreApplication> #include<iostream> #include<cstdio> #include<cstring> #include<cstdlib> #include<cmath> #include<algorithm> #define inf 0x7fffffff using namespace std; void* operator new(size_t sz) { printf("operator new: %d Bytes\n",sz); void* m = malloc(sz); if (!m) puts("out of memory"); return m; } void* operator new [](size_t sz) { printf("operator new: %d Bytes\n",sz); void* m = malloc(sz); if (!m) puts("out of memory"); return m; } void operator delete(void* m) { puts("operator delete"); free(m); } void operator delete[](void* m) { puts("operator delete"); free(m); } class S { public: S() {puts("S::S()"); } ~S() {puts("S::~S()"); } private: int an[1000]; }; void func() { puts("creating & destroying an int"); int* q = new int(23); delete q; puts("creating & destroying an int[]"); int* p = new int[10](); delete []p; puts("creating & destroying an s"); S* s = new S; delete s; puts("creating & destroying S[3]"); S* sa = new S[3]; delete []sa; } int main(int argc, char *argv[]) { QCoreApplication a(argc, argv); func(); return a.exec(); }
輸出：
creating & destroying an int operator new: 4 Bytes operator delete creating & destroying an int[] operator new: 40 Bytes operator delete creating & destroying an s operator new: 4000 Bytes S::S() S::~S() operator delete creating & destroying S[3] operator new: 12004 Bytes S::S() S::S() S::S() S::~S() S::~S() S::~S() operator delete
對於一個類重載new和delete
為一個類重載new和delete的時候，盡管不必顯式的使用static，但是實際上仍是在創建static成員函數。它的語法也和重載任何其它運算符一樣。當編譯器看到使用new創建自己定義的類的對象時，它選擇成員版本的operator new()而不是全局版本的new()。但是全局版本的new和delete仍為所有其他類型對象使用（除非它們也有自己的new和delete）。這個和全局變量、局部變量的意思是一樣的。
#include<iostream> #include<cstddef> #include<fstream> #include<new> using namespace std; ofstream out("Framis.out"); class Framis { public: enum{psize = 100 }; Framis() {out<< "Framis()" <<endl; } ~Framis() {out<< "~Framis() ... " <<endl; } void* operator new(size_t) throw (bad_alloc); void operator delete(void*); private: enum{sz = 10 }; char c[sz]; static unsigned char pool[]; static bool alloc_map[]; }; unsigned char Framis::pool[psize*sizeof(Framis)]; bool Framis::alloc_map[psize]={false}; void* Framis::operator new(size_t sz) throw(bad_alloc) { for (int i=0; i<psize; ++i) { if (!alloc_map[i]) { out<< "using block " << i << " ... "; alloc_map[i]=true; return pool+(i*sizeof(Framis)); } } out<< "out of memory" <<endl; throw bad_alloc(); } void Framis::operator delete(void* m) { if (!m) return; unsigned long block = (unsigned long)m-(unsigned long)pool; block /= sizeof(Framis); out<< "freeing block " << block <<endl; alloc_map[block]=false; } int main() {
cout << sizeof(Framis) << endl; Framis* f[Framis::psize]; try { for (int i=0; i<Framis::psize; i++) { f[i]=new Framis; // new 的時候先跑進 new 重載函數中，再調用構造函數 } new Framis; } catch(bad_alloc) { cerr<< "Out of memory!" <<endl; } delete f[10]; f[10]=0; Framis* X=new Framis; delete X;// delete 的時候先調用析構函數，再跑進delete重載函數中 for (int j=0; j<Framis::psize; j++) { delete f[j]; } return 0; }
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Framis() using block 70 ... Framis() using block 71 ... Framis() using block 72 ... Framis() using block 73 ... Framis() using block 74 ... Framis() using block 75 ... Framis() using block 76 ... Framis() using block 77 ... Framis() using block 78 ... Framis() using block 79 ... Framis() using block 80 ... Framis() using block 81 ... Framis() using block 82 ... Framis() using block 83 ... Framis() using block 84 ... Framis() using block 85 ... Framis() using block 86 ... Framis() using block 87 ... Framis() using block 88 ... Framis() using block 89 ... Framis() using block 90 ... Framis() using block 91 ... Framis() using block 92 ... Framis() using block 93 ... Framis() using block 94 ... Framis() using block 95 ... Framis() using block 96 ... Framis() using block 97 ... Framis() using block 98 ... Framis() using block 99 ... Framis() out of memory Out of memory! ~Framis() ... freeing block 10 using block 10 ... 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為數組重載new和delete
上一段文字中我們講到如果為一個類重載operator new()和operator delete()，那么無論何時創建這個類的一個對象都將調用這些運算符。但是如果要創建這個類的一個對象數組的時候，全局operator new()就會被立即調用，用來為這個數組分配足夠的內存。對此，我們可以通過為這個類重載運算符的數組版本，即operator new[]和operator delete[]，來控制對象數組的內存分配。
#include<iostream> #include<fstream> #include<new> using namespace std; ofstream trace("ArrayOperatorNew.out"); class Widget { public: Widget() {trace<< "*" <<endl; } ~Widget() {trace<< "~" <<endl; } void* operator new(size_t sz) { trace<< "Widget::new: " << sz << " byte" <<endl; return ::new char[sz]; } void operator delete(void* p) { trace<< "Widget::delete" <<endl; ::delete []p; } void* operator new[](size_t sz) { trace<< "Widget::new[]: " << sz << " bytes" <<endl; return ::new char[sz]; } void operator delete[](void* p) { trace<< "Widget::delete[]" <<endl; ::delete []p; } private: enum{sz=10 }; int an[sz]; }; int main() { trace<< "new Widget" <<endl; Widget* w=new Widget; trace<<endl<< "delete Widget" <<endl; delete w; trace<<endl<< "new Widget[25]" <<endl; Widget* wa=new Widget[25]; trace<<endl<< "delete []Widget" <<endl; delete []wa; return 0; }
new Widget Widget::new: 40 byte * delete Widget ~ Widget::delete new Widget[25] Widget::new[]: 1004 bytes * * * * * * * * * * * * * * * * * * * * * * * * * delete []Widget ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Widget::delete[]
C++空類的大小是1