C++ Primer 5th笔记（chap 12）动态内存 allocator类

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1. 标准库allocator类及其算法

注意：

    const size_t n = 100;
    allocator<string> allocStr;      // object that can allocate strings
    auto p = allocStr.allocate(n);   // allocate n unconstructed strings

    //cout << *p << endl;

    auto q = p; // q will point to one past the last constructed element
    allocStr.construct(q++);         // *q is the empty string
    cout << *(q - 1) << endl;

    allocStr.construct(q++, 10, 'c'); // *q is cccccccccc
    cout << *(q - 1) << endl;

    //allocStr.construct(q++, "hi");    // *q is hi!
    //cout << *(q - 1) << endl;

    int nCount = 0;
    cout << *p << endl;  // ok: uses the string output operator
    while (q != p) {
        nCount++;
        allocStr.destroy(--q);  // free the strings we actually allocated
    }
    cout << nCount << endl;  // ok: uses the string output operator 

    allocStr.deallocate(p, n);  // return the memory we allocated

    p = allocStr.allocate(n);   // allocate n unconstructed strings
    string s;
    q = p;                   // q points to the memory for first string
    ifstream in("E:/temp/storyDataFile");
    while (in >> s && q != p + n)
        allocStr.construct(q++, s); // construct only as many strings as we need
    size_t size = q - p;         // remember how many strings we read

    // use the array
    cout << "read " << size << " strings" << endl;

    for (q = p + size - 1; q != p; --q)
        allocStr.destroy(q);         // free the strings we allocated
    allocStr.deallocate(p, n);       // return the memory we allocated

    in.close();
    in.open("E:/temp/storyDataFile");
    p = new string[n];            // construct n empty strings
    q = p;                        // q points to the first string
    while (in >> s && q != p + n)
        *q++ = s;                 // assign a new value to *q
    size = q - p;                 // remember how many strings we read

    cout << "read " << size << " strings" << endl;

2. "copy和填充未初始化的内存"算法

使用示例：

vector<int> vi{ 1,2,3,4,5,6,7,8,9 };
  
allocator<int> alloc;

// allocate twice as many elements as vi holds
auto p = alloc.allocate(vi.size() * 2);

// construct elements starting at p as copies of elements in vi
auto q = uninitialized_copy(vi.begin(), vi.end(), p);
 
// initialize the remaining elements to 42
uninitialized_fill_n(q, vi.size(), 42);

for (size_t i = 0; i != vi.size(); ++i)
cout << *(p + i) << " ";
cout << endl;

for (size_t i = 0; i != vi.size(); ++i)
cout << *(q + i) << " ";
cout << endl;

alloc.deallocate(p, vi.size());

retsult:

1 2 3 4 5 6 7 8 9
42 42 42 42 42 42 42 42 42

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