C++内存管理11 重载new()和delete()/下
Foo* pf=new(300,'c') Foo;
这个叫placement new但不定点。
重载时第一个参数必须是size_t
因为正常new Foo时会把Foo大小传给operator new
1.重载new()
class Foo
{
public:
Foo() { cout << "Foo::Foo()" << endl; }
Foo(int) {
cout << "Foo::Foo(int)" << endl;
// throw Bad();
}
//(1) 這個就是一般的 operator new() 的重載
void* operator new(size_t size) {
cout << "operator new(size_t size), size= " << size << endl;
return malloc(size);
}
//(2) 這個就是標準庫已經提供的 placement new() 的重載 (形式)
// (所以我也模擬 standard placement new 的動作, just return ptr)
void* operator new(size_t size, void* start) {
cout << "operator new(size_t size, void* start), size= " << size << " start= " << start << endl;
return start;
}
//(3) 這個才是嶄新的 placement new
void* operator new(size_t size, long extra) {
cout << "operator new(size_t size, long extra) " << size << ' ' << extra << endl;
return malloc(size+extra);
}
//(4) 這又是一個 placement new
void* operator new(size_t size, long extra, char init) {
cout << "operator new(size_t size, long extra, char init) " << size << ' ' << extra << ' ' << init << endl;
return malloc(size+extra);
}
//(5) 這又是一個 placement new, 但故意寫錯第一參數的 type (它必須是 size_t 以滿足正常的 operator new)
//这里会报错
//! void* operator new(long extra, char init) { //[Error] 'operator new' takes type 'size_t' ('unsigned int') as first parameter [-fpermissive]
//! cout << "op-new(long,char)" << endl;
//! return malloc(extra);
//! }
2.重载delete()
//搭配上述placement new的placement delete
void operator delete(void*,size_t)
{ cout << "operator delete(void*,size_t) " << endl; }
//(2) 這是對應上述的 (2)
void operator delete(void*,void*)
{ cout << "operator delete(void*,void*) " << endl; }
//(3) 這是對應上述的 (3)
void operator delete(void*,long)
{ cout << "operator delete(void*,long) " << endl; }
//(4) 這是對應上述的 (4)
//如果沒有一一對應, 也不會有任何編譯報錯
void operator delete(void*,long,char)
{ cout << "operator delete(void*,long,char) " << endl; }
test代码:
void test_overload_placement_new()
{
cout << "\n\n\ntest_overload_placement_new().......... \n";
Foo start; //Foo::Foo
Foo* p1 = new Foo; //op-new(size_t)
Foo* p2 = new (&start) Foo; //op-new(size_t,void*)
Foo* p3 = new (100) Foo; //op-new(size_t,long)
Foo* p4 = new (100,'a') Foo; //op-new(size_t,long,char)
//带括号的new不是一般的placement new所以调用第3个重载的operator new
//new(100)的100是extra，size是4因为私有一个int成员。
Foo* p5 = new (100) Foo(1); //op-new(size_t,long) op-del(void*,long)
Foo* p6 = new (100,'a') Foo(1); //
Foo* p7 = new (&start) Foo(1); //
Foo* p8 = new Foo(1); //
//VC6 warning C4291: 'void *__cdecl Foo::operator new(unsigned int)'
//no matching operator delete found; memory will not be freed if
//initialization throws an exception
}
运行结果:
最后附上完整代码：
#include
#include
#include //for test
using namespace std;
namespace jj07
{
class Bad { };
class Foo
{
public:
Foo() { cout << "Foo::Foo()" << endl; }
Foo(int) {
cout << "Foo::Foo(int)" << endl;
// throw Bad();
}
//(1) 這個就是一般的 operator new() 的重載
void* operator new(size_t size) {
cout << "operator new(size_t size), size= " << size << endl;
return malloc(size);
}
//(2) 這個就是標準庫已經提供的 placement new() 的重載 (形式)
// (所以我也模擬 standard placement new 的動作, just return ptr)
void* operator new(size_t size, void* start) {
cout << "operator new(size_t size, void* start), size= " << size << " start= " << start << endl;
return start;
}
//(3) 這個才是嶄新的 placement new
void* operator new(size_t size, long extra) {
cout << "operator new(size_t size, long extra) " << size << ' ' << extra << endl;
return malloc(size+extra);
}
//(4) 這又是一個 placement new
void* operator new(size_t size, long extra, char init) {
cout << "operator new(size_t size, long extra, char init) " << size << ' ' << extra << ' ' << init << endl;
return malloc(size+extra);
}
//(5) 這又是一個 placement new, 但故意寫錯第一參數的 type (它必須是 size_t 以滿足正常的 operator new)
//! void* operator new(long extra, char init) { //[Error] 'operator new' takes type 'size_t' ('unsigned int') as first parameter [-fpermissive]
//! cout << "op-new(long,char)" << endl;
//! return malloc(extra);
//! }
//以下是搭配上述 placement new 的各個 called placement delete.
//當 ctor 發出異常，這兒對應的 operator (placement) delete 就會被喚起.
//應該是要負責釋放其搭檔兄弟 (placement new) 分配所得的 memory.
//(1) 這個就是一般的 operator delete() 的重載
void operator delete(void*,size_t)
{ cout << "operator delete(void*,size_t) " << endl; }
//(2) 這是對應上述的 (2)
void operator delete(void*,void*)
{ cout << "operator delete(void*,void*) " << endl; }
//(3) 這是對應上述的 (3)
void operator delete(void*,long)
{ cout << "operator delete(void*,long) " << endl; }
//(4) 這是對應上述的 (4)
//如果沒有一一對應, 也不會有任何編譯報錯
void operator delete(void*,long,char)
{ cout << "operator delete(void*,long,char) " << endl; }
private:
int m_i;
};
//-------------
void test_overload_placement_new()
{
cout << "\n\n\ntest_overload_placement_new().......... \n";
Foo start; //Foo::Foo
Foo* p1 = new Foo; //op-new(size_t)
Foo* p2 = new (&start) Foo; //op-new(size_t,void*)
Foo* p3 = new (100) Foo; //op-new(size_t,long)
Foo* p4 = new (100,'a') Foo; //op-new(size_t,long,char)
Foo* p5 = new (100) Foo(1); //op-new(size_t,long) op-del(void*,long)
Foo* p6 = new (100,'a') Foo(1); //
Foo* p7 = new (&start) Foo(1); //
Foo* p8 = new Foo(1); //
//VC6 warning C4291: 'void *__cdecl Foo::operator new(unsigned int)'
//no matching operator delete found; memory will not be freed if
//initialization throws an exception
}
} //namespace
int main(){
jj07::test_overload_placement_new();
return 0;
}
如果存在Bad()，抛出异常，当placement new调用到这个函数时，即调用相应的delete()进行释放，目的是为了防止内存分配好了后对象建立不起来，写出抛出异常可以进行释放。
Foo(int) {
cout << "Foo::Foo(int)" << endl;
throw Bad();//这句起作用
}
运行结果如下：
即使new()和delete()没有一一对应，也不会报错。你的意思是放弃处理构造函数发出的异常。
例子：
平时用的string是一个type define 的basic_string
每创建一个字符多一个extra。
以上来自侯捷老师视频仅用于学习。