我有一个成员函数,可打印boost :: fibonacci_heap的快照
virtual void printSnapshot(std::ostream& ss) {
Heap heap(this->heap);
double prev_price = DBL_MAX;
while(heap.size() > 0) {
const Order& order = heap.top();
if(order.price != prev_price) {
if(prev_price != DBL_MAX) ss << std::endl;
ss << order.price << " | ";
}
ss << order.quantity << " ";
prev_price = order.price;
heap.pop();
}
ss << std::endl;
}
我在另一个成员函数中调用此成员函数,
while(std::getline(stream, line)) {
... // do something on this->heap.
this->printSnapshot(std::cout);
}
由于堆是通过“ printSnapshot”开头的副本构造函数创建的,因此“ printSnapshot”应更改this-> heap。但是,此程序导致段错误,而以下情况则不会:
while(std::getline(stream, line)) {
... // do something on this->heap.
// this->printSnapshot(std::cout);
}
现在,如果我们在sprintSnapshot的定义中添加一个const关键字,即
virtual void printSnapshot(std::ostream& ss) const {
Heap heap(this->heap);
double prev_price = DBL_MAX;
while(heap.size() > 0) {
const Order& order = heap.top();
if(order.price != prev_price) {
if(prev_price != DBL_MAX) ss << std::endl;
ss << order.price << " | ";
}
ss << order.quantity << " ";
prev_price = order.price;
heap.pop();
}
ss << std::endl;
}
段故障消失。怎么解释呢?
fibonacci_heap
带有lvalue reference
(非const)的构造函数显然没有做正确的事情。
它没有记录应执行的操作:http : //www.boost.org/doc/libs/1_55_0/doc/html/boost/heap/fibonacci_heap.html#idp21129704-bb
我认为这可能是一个可报告的错误。我会对此进行调查。
更新令人惊讶的是,此构造方法的行为显然等同于move-construction:
#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES /// \copydoc boost::heap::priority_queue::priority_queue(priority_queue &&) fibonacci_heap(fibonacci_heap && rhs): super_t(std::move(rhs)), top_element(rhs.top_element) { roots.splice(roots.begin(), rhs.roots); rhs.top_element = NULL; } fibonacci_heap(fibonacci_heap & rhs): super_t(rhs), top_element(rhs.top_element) { roots.splice(roots.begin(), rhs.roots); rhs.top_element = NULL; }
后者有一个奇怪的副作用,就是简单地从原始(侵入式)列表中删除所有根。这看起来像一个明确的错误。
只需删除此构造函数即可使代码正常工作。
基本的解决方法是避免使用lvalue-ref构造函数:
Heap cloned(static_cast<Heap const&>(this->heap));
同时,这是一个自包含的复制器:
#include <boost/heap/fibonacci_heap.hpp>
#include <iostream>
#include <random>
namespace {
#undef DBL_MAX
static double DBL_MAX = std::numeric_limits<double>::max();
std::mt19937 rng;
//std::uniform_real_distribution<double> dist(100, 4000);
std::discrete_distribution<int> dist({1,1,1,1,1,1});
static auto price_gen = [&] {
static double values[] = {52.40, 12.30, 87.10, 388., 0.10, 23.40};
return values[dist(rng)];
};
}
struct Order {
double price = price_gen();
unsigned quantity = rand() % 4 + 1;
double subtotal() const { return price * quantity; }
bool operator<(Order const& other) const { return subtotal() < other.subtotal(); }
};
using Heap = boost::heap::fibonacci_heap<Order>;
struct Y {
virtual void printSnapshot(std::ostream &ss) {
//Heap cloned(static_cast<Heap const&>(this->heap));
Heap cloned(this->heap);
double prev_price = DBL_MAX;
while (cloned.size() > 0) {
const Order &order = cloned.top();
if (order.price != prev_price) {
if (prev_price != DBL_MAX)
ss << std::endl;
ss << order.price << " | ";
}
ss << order.quantity << " ";
prev_price = order.price;
cloned.pop();
}
ss << std::endl;
}
void generateOrders() {
for (int i=0; i<3; ++i) {
heap.push({});
}
}
Heap heap;
};
int main() {
Y y;
for(int i=0; i<10; ++i) {
y.generateOrders();
y.printSnapshot(std::cout);
}
}
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