[英]Use std::remove_if on a std::vector filled with pointers
我对std::remove_if
函数有一个小问题。 我的程序中某处内存泄漏,我怀疑erase
功能以某种方式被破坏。
实际上,我的代码中有这个
std::vector<Object*> v; // Just the constructor to show you
v.erase(std::remove_if(begin(v), end(v), [Foo f](Object *o){
return o->containsFoo(f);
}), end(v));
但是经过一些研究,这比以前更好吗?
v.erase(std::remove_if(begin(v), end(v), [Foo f](Object *o){
if(o->containsFoo(f)) {
delete o;
return true;
}
return false;
}), end(v));
还是我应该使用其他东西?
您确实应该使用智能指针而不是裸Object*
-要么
std::vector<std::unique_ptr<int>>
要么
std::vector<std::shared_ptr<int>>
以适当者为准。 如果使用裸C风格的指针,那么错过关键的delete
(或delete
两次)太容易了。
尽管如此,很容易看到一种方法泄漏而另一种方法没有泄漏:
#include <algorithm>
#include <vector>
int main(int argc, char **)
{
std::vector<int*> v{ new int(1), new int(-1) };
if (argc < 2) {
// First version
v.erase(std::remove_if(begin(v), end(v),
[](int *o){
return *o < 0;
}),
end(v));
} else {
// Second version
v.erase(std::remove_if(begin(v), end(v),
[](int *o){
if (*o < 0) {
delete o;
return true;
}
return false;
}),
end(v));
}
// normal cleanup
for (int *p: v)
delete p;
}
我在运行时不带参数(调用第一个版本),然后带参数(调用第二个版本)。 看看会发生什么:
g++ -std=c++11 -g -Wall -Wextra 34191606.cpp -o 34191606
valgrind --leak-check=full ./34191606
==16894== HEAP SUMMARY:
==16894== in use at exit: 72,708 bytes in 2 blocks
==16894== total heap usage: 4 allocs, 2 frees, 72,728 bytes allocated
==16894==
==16894== 4 bytes in 1 blocks are definitely lost in loss record 1 of 2
==16894== at 0x4C2B0E0: operator new(unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==16894== by 0x400881: main (34191606.cpp:6)
==16894==
==16894== LEAK SUMMARY:
==16894== definitely lost: 4 bytes in 1 blocks
==16894== indirectly lost: 0 bytes in 0 blocks
==16894== possibly lost: 0 bytes in 0 blocks
==16894== still reachable: 72,704 bytes in 1 blocks
==16894== suppressed: 0 bytes in 0 blocks
==16894== Reachable blocks (those to which a pointer was found) are not shown.
==16894== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
valgrind --leak-check=full ./34191606 -
==16895== HEAP SUMMARY:
==16895== in use at exit: 72,704 bytes in 1 blocks
==16895== total heap usage: 4 allocs, 3 frees, 72,728 bytes allocated
==16895==
==16895== LEAK SUMMARY:
==16895== definitely lost: 0 bytes in 0 blocks
==16895== indirectly lost: 0 bytes in 0 blocks
==16895== possibly lost: 0 bytes in 0 blocks
==16895== still reachable: 72,704 bytes in 1 blocks
==16895== suppressed: 0 bytes in 0 blocks
==16895== Reachable blocks (those to which a pointer was found) are not shown.
==16895== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
请注意,在第一个版本中,您永远不会删除从矢量中删除了指针的对象,并且该对象被报告为泄漏的。 在第二个版本中,没有泄漏的内存。
lambda 更好 ,但是如果我是您(并且由于某些原因您不准备使用std::unique_ptr
),我会这样做:
const auto predicate = [Foo f](Object *o){return !o->containsFoo(f);};
const auto new_end = std::partition(begin(v), end(v), predicate);
std::for_each(new_end, end(v), [](Object *o){delete o;});
v.erase(new_end,end(v));
换一种说法:
remove_if
partition
,将不需要的指针拖到最后 for_each
删除所有不需要的对象 erase
摆脱(现在无效)不需要的指针。 关键是,在事物四处移动时删除指针是很麻烦的。
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