如何将 STL 算法与指针向量结合使用

发布于 2024-07-19 06:16:59 字数 512 浏览 3 评论 0原文

我有一个不属于容器的指针向量。如何对指针的目标使用算法。我尝试使用 boost 的 ptr_vector，但它会在超出范围时尝试删除指针。

这是一些需要工作的代码：

vector<int*> myValues;
// ... myValues is populated
bool consistent = count(myValues.begin(), myValues.end(), myValues.front()) == myValues.size();
auto v = consistent ? myValues.front() : accumulate(myValues.begin(), myValues.end(), 0) / myValues.size();
fill(myValues.begin(), myValues.end(), v);
// etc.

我意识到 for 循环可以工作，但是这种情况发生在很多地方，所以某种一元适配器？我没能找到一个。提前致谢！

原文

I have a vector of pointers that are not owned by the container. How do I use algorithms on the targets of the pointers. I tried to use boost's ptr_vector, but it tries to delete the pointers when it goes out of scope.

Here is some code that needs to work:

vector<int*> myValues;
// ... myValues is populated
bool consistent = count(myValues.begin(), myValues.end(), myValues.front()) == myValues.size();
auto v = consistent ? myValues.front() : accumulate(myValues.begin(), myValues.end(), 0) / myValues.size();
fill(myValues.begin(), myValues.end(), v);
// etc.

I realize that for loops would work, but this happens in a bunch of places, so some kind of unary adapter? I wasn't able to find one. Thanks in advance!

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诗酒趁年少 2024-07-26 06:16:59

您可以使用 Boost 间接迭代器。当取消引用时（使用operator*()），它会应用一个额外的取消引用，因此最终会得到迭代器引用的指针所指向的值。有关更多信息，您还可以查看这个关于取消引用迭代器的问题。

这是一个简单的例子：

std::vector<int*> vec;

vec.push_back(new int(1));
vec.push_back(new int(2));

std::copy(boost::make_indirect_iterator(vec.begin()),
          boost::make_indirect_iterator(vec.end()),
          std::ostream_iterator<int>(std::cout, " "));     // Prints 1 2

You could use Boost Indirect Iterator. When dereferenced (with operator*() ), it applies an extra dereference, so you end up with the value pointed by the pointer referenced by the iterator. For more information, you can also see this question about a dereference iterator.

Here's a simple example:

std::vector<int*> vec;

vec.push_back(new int(1));
vec.push_back(new int(2));

std::copy(boost::make_indirect_iterator(vec.begin()),
          boost::make_indirect_iterator(vec.end()),
          std::ostream_iterator<int>(std::cout, " "));     // Prints 1 2

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缱倦旧时光 2024-07-26 06:16:59

bool consistent = count_if(myValues.begin(), myValues.end(), 
   bind2nd(ptr_fun(compare_ptr), *myValues.front())) == myValues.size();

int v = consistent ? *myValues.front() : accumulate(
   myValues.begin(), myValues.end(), 0, sum_int_ptr) / myValues.size();

for_each(myValues.begin(), myValues.end(), bind1st(ptr_fun(assign_ptr),v));

Fill 不能接受分配函数（这样它就会取消引用指针）。因此使用了 for_each() 。为了优化，明智的做法是在运行 for_each() 之前添加 if(!consistent)。上述STL oneliner中使用的函数：

int sum_int_ptr(int total, int * a) { return total + *a; }    
void assign_ptr(int v, int *ptr) { *ptr = v; }    
bool compare_ptr(int* a, int pattern) { return *a == pattern; }

bool consistent = count_if(myValues.begin(), myValues.end(), 
   bind2nd(ptr_fun(compare_ptr), *myValues.front())) == myValues.size();

int v = consistent ? *myValues.front() : accumulate(
   myValues.begin(), myValues.end(), 0, sum_int_ptr) / myValues.size();

for_each(myValues.begin(), myValues.end(), bind1st(ptr_fun(assign_ptr),v));

Fill can't take assign function (so that it would dereference pointers). Therefore for_each() was used. For optimization it would be wise to add if(!consistent) before running for_each(). Functions used in above STL one liners:

int sum_int_ptr(int total, int * a) { return total + *a; }    
void assign_ptr(int v, int *ptr) { *ptr = v; }    
bool compare_ptr(int* a, int pattern) { return *a == pattern; }

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