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对未初始化内存的操作（Operations on uninitialized memory）

一、uninitialized_copy

1、原型：

template< class InputIt, class ForwardIt >ForwardIt uninitialized_copy( InputIt first, InputIt last, ForwardIt d_first );

2、说明：

将对象范围复制到未初始化的内存区域

3、官方demo

#include 
   
    #include 
    
     #include 
     
      #include 
      
        int main(){
       const char *v[] = {
   "This", "is", "an", "example"};     auto sz = std::size(v);     if(void *pbuf = std::aligned_alloc(alignof(std::string), sizeof(std::string) * sz)) {
           try {
               auto first = static_cast
       
        (pbuf);            auto last = std::uninitialized_copy(std::begin(v), std::end(v), first);             for (auto it = first; it != last; ++it)                std::cout << *it << '_';             std::destroy(first, last);        } catch(...) {
   }        std::free(pbuf);    }}
       
      
     
    
   

Output:

This_is_an_example_

二、uninitialized_fill

1、原型：

template< class ForwardIt, class T >void uninitialized_fill( ForwardIt first, ForwardIt last, const T& value );

2、说明：

以指定值value填充。

3、官方demo

#include 
   
    #include 
    
     #include 
     
      #include 
      
       #include 
       
         int main(){
       std::string* p;    std::size_t sz;    std::tie(p, sz) = std::get_temporary_buffer
        
         (4); std::uninitialized_fill(p, p+sz, "Example"); for (std::string* i = p; i != p+sz; ++i) { std::cout << *i << '\n'; i->~basic_string
         
          (); } std::return_temporary_buffer(p);}
         
        
       
      
     
    
   

Output:

ExampleExampleExampleExample

三、uninitialized_fill_n

1、原型：

template< class ForwardIt, class Size, class T >ForwardIt uninitialized_fill_n( ForwardIt first, Size count, const T& value );

2、说明：

以指定值value填充从first开始count个数据。

3、官方demo

#include 
   
    #include 
    
     #include 
     
      #include 
      
       #include 
       
         int main(){
       std::string* p;    std::size_t sz;    std::tie(p, sz) = std::get_temporary_buffer
        
         (4); std::uninitialized_fill_n(p, sz, "Example"); for (std::string* i = p; i != p+sz; ++i) { std::cout << *i << '\n'; i->~basic_string
         
          (); } std::return_temporary_buffer(p);}
         
        
       
      
     
    
   

Output:

ExampleExampleExampleExample

C库（C library）

一、qsort

1、原型：

void qsort( void *ptr, std::size_t count, std::size_t size, *comp );

2、说明：

按升序对ptr的数组进行排列

3、官方demo

#include 
   
    #include 
    
     #include 
     
       int main(){
       int a[] = {
   -2, 99, 0, -743, 2, INT_MIN, 4};    constexpr std::size_t size = sizeof a / sizeof *a;     std::qsort(a, size, sizeof *a, [](const void* a, const void* b){
           int arg1 = *static_cast
      
       (a);        int arg2 = *static_cast
       
        (b);         if(arg1 < arg2) return -1;        if(arg1 > arg2) return 1;        return 0;    });     for(int ai : a)        std::cout << ai << ' ';}
       
      
     
    
   

Output:

-2147483648 -743 -2 0 2 4 99

二、bsearch

1、原型：

void* bsearch( const void* key, const void* ptr, std::size_t count,           std::size_t size, *comp );

2、说明：

二分查找：在ptr数组中找到key

3、官方demo

#include 
   
    #include 
    
      int compare(const void *ap, const void *bp){
       const int *a = (int *) ap;    const int *b = (int *) bp;    if(*a < *b)        return -1;    else if(*a > *b)        return 1;    else        return 0;} int main(int argc, char **argv){
       const int ARR_SIZE = 8;    int arr[ARR_SIZE] = {
    1, 2, 3, 4, 5, 6, 7, 8 };     int key1 = 4;    int *p1 = (int *) std::bsearch(&key1, arr, ARR_SIZE, sizeof(arr[0]), compare);    if(p1)        std::cout << "value " << key1 << " found at position " << (p1 - arr) << '\n';     else        std::cout << "value " << key1 << " not found\n";     int key2 = 9;    int *p2 = (int *) std::bsearch(&key2, arr, ARR_SIZE, sizeof(arr[0]), compare);    if(p2)        std::cout << "value " << key2 << " found at position " << (p2 - arr) << '\n';     else        std::cout << "value " << key2 << " not found\n";}
    
   

Output:

value 4 found at position 3value 9 not found

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