placement new操作符能夠在分配內(nèi)存時指定內(nèi)存位置。下面的程序使用了placement new操作符和常規(guī)new操作符給對象分配內(nèi)存。

// placenew.cpp -- new, placement new, no delete #include <iostream> #include <string> #include <new>using namespace std; const int BUF = 512;class JustTesting { private:string words;int number; public:JustTesting(const string &s = "Just Testing", int n = 0){words = s; number = n; cout << words << " constructed\n";}~JustTesting() { cout << words << " destroyed\n"; }void Show() const { cout << words << ", " << number << endl; } };int main(void) {char *buffer = new char [BUF]; // get a block of memoryJustTesting *pc1, *pc2;pc1 = new (buffer)JustTesting; // place object in bufferpc2 = new JustTesting("heap1", 20); // place object on heap cout << "Memory block address:\n" << "buffer: "<< (void *)buffer << " heap: " << pc2 << endl;cout << "Memory contents: \n";cout << pc1 << ": ";pc1->Show();cout << pc2 << ": ";pc2->Show();JustTesting *pc3, *pc4;pc3 = new (buffer) JustTesting("bad Idea", 6);pc4 = new JustTesting("Heap2", 10);cout << "Memory contents: \n";cout << pc3 << ": ";pc3->Show();cout << pc4 << ": ";pc4->Show();delete pc2; // free heap1delete pc4; // free heap2delete [] buffer; // free buffercout << "Done\n";return 0; }

執(zhí)行結(jié)果：

[root@localhost 桌面]# ./new Just Testing constructed heap1 constructed Memory block address: buffer: 0x936a008 heap: 0x936a248 Memory contents: 0x936a008: Just Testing, 0 0x936a248: heap1, 20 bad Idea constructed Heap2 constructed Memory contents: 0x936a008: bad Idea, 6 0x936a290: Heap2, 10 heap1 destroyed Heap2 destroyed Done

上面的程序使用placement new操作時存在兩個問題。首先，在創(chuàng)建第二個對象時，placement new操作符使用一個新對象來覆蓋用于第一個對象的內(nèi)存單元。顯然，如果類動態(tài)地為其成員分配內(nèi)存，這將引發(fā)問題。

???? 其次，將delete用于pc2和pc4時，將自動調(diào)用為pc2和pc4指向的對象調(diào)用析構(gòu)函數(shù)；然而，將delete[]用于buffer時，不會為使用布局new操作符創(chuàng)建的對象調(diào)用析構(gòu)函數(shù)。

?? 為確定兩個單元不重疊，可以這樣做：

pc1 = new (buffer) JustTesting; pc3 = new (buffer + sizeof(JustTesting)) JustTesting("Better Idea", 6);

?其中指針pc3相對于pc1的偏移量為JustTesting對象的大小

? 第二個教訓是，如果使用placement new操作符來為對象分配內(nèi)存，必須確保其析構(gòu)函數(shù)被調(diào)用，但如何確保呢？

? 例如，在堆中創(chuàng)建的對象，可以這樣做：

delete pc2;

然而，對于使用placement new操作符創(chuàng)建的對象，不能像下面一樣調(diào)用delete

delete pc1; // NO!!!

? 原因在于delete可與常規(guī)new操作符配合使用，但不能與placement new操作符配合使用。

那么我們要顯示調(diào)用析構(gòu)函數(shù)，必須指定要銷毀的對象：

pc3->~JustTesting();?? // destroy object pointed to by pc3

int main(void) {char *buffer = new char[BUF]; // get a block of memoryJustTesting *pc1, *pc2;pc1 = new (buffer) JustTesting; // place object in bufferpc2 = new JustTesting("Heap1", 20); // place object on heap cout << "Memory block addresses: /n" << "buffer: "<< (void *)buffer << " heap: " << pc2 << endl;cout << "Memory contents: ";cout << pc1 << ": ";pc1->Show();cout << pc2 << ": ";pc2->Show();JustTesting *pc3, *pc4;// fix placement new location pc3 = new (buffer + sizeof(JustTesting)) JustTesting("better Idea", 6);pc4 = new JustTesting("Heap2", 10);cout << "Memory contents: ";cout << pc3 << ": ";pc3->Show();cout << pc4 << ": ";pc4->Show();delete pc2; // free heap1delete pc4; // free heap2// explicitly destroy placement new object pc3->~JustTesting(); // destroy object pointed to by pc3pc1->~JustTesting(); // destroy object pointed to by pc1delete []buffer; // free buffercout << "Done/n";return 0; }

?

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