我想同时插入几个STL容器中,要么全部成功,要么全部保持不变。我该如何优雅地做到这一点?
在我的示例中,我有一个带有各种STL容器作为数据成员的自定义容器。插入操作将插入到各种容器中,但是如果其中一个失败,则需要撤消更改以确保所有不变式仍然有效。我该怎么做(很好)?
我的第一个直觉是退回所有插入的内容并撤消它们。尽管我希望我的例子能说明这一点,但这似乎有些过分。
示例(很抱歉,如果不够“最小”):
struct DataItem {
int x;
float y;
char c;
};
class MyContainer {
public:
void Insert(std::vector<std::string> names,
std::unordered_set<int> categories,
DataItem data);
private:
/* invariant: if a string 'name' is mapped to an id 'i' in 'name_to_id_'
it must be the position of a 'DataItem' object in 'data_'
and contained in at least one of the categories */
std::map<std::string, int> name_to_id_;
std::map<int, std::unordered_set<int>> categories_;
std::vector<DataItem> data_;
};
MyContainer::Insert(std::vector<std::string> names,
std::unordered_set<Categories> categories,
DataItem data) {
/* ensure names, and categories are not empty and data is valid */
int id = data_.size();
for (auto it = names.begin(); it != names.end(); ++it) {
if (this->name_to_id_.count(name)) {
/* Clean up all names inserted so far by iterating backwards to names.begin()
using this->name_to_id_.erase */
throw SomeException("Insertion failed");
}
try {
this->name_to_id_.insert({*it, id});
} catch (/* what do I catch here? */) {
/* Clean up all names inserted so far by iterating backwards to names.begin()
using this->name_to_id_.erase */
throw SomeException("Insertion failed");
}
}
for (auto it = categories.begin(); it != categories.end(); ++it) {
try {
this->categories_.at(*it).insert(id);
} catch (/* what do I catch here? */) {
/* Clean up all names and all categories id was inserted into so far by
iterating backwards to categories.begin() using
this->categories_.at(*it).erase(id) */
throw SomeException("Insertion failed");
}
}
try {
this->data_.push_back(data);
} catch (/* what do I catch here? */) {
/* Once again clean up all categories and names... */
throw SomeException("Insertion failed");
}
}
即使没有写出清理内容,这似乎也是多余的。特别是考虑到insert
和push_back
很少应该以开头失败。这真的是我需要做的吗?另外,擦除更改的最安全方法是我可以确定
std::unordered_set
和std::map
的组合是find
和erase
,但我找不到关于find
的异常安全性的任何信息。它总是成功吗?我突然想到,必须将
insert
和push_back
语句包装在try
块中才能真正处理该异常,但是我能捕获什么异常? 最佳答案
我没有提到实际的清理操作,也没有最有效的方法。您将需要记录在什么阶段插入失败并相应地进行。
struct DataItem {
int x;
float y;
char c;
};
class SomeException : std::exception
{
public:
SomeException(char *c) : std::exception(c) {};
};
class MyContainer {
public:
void Insert(std::vector<std::string> names,
std::unordered_set<int> categories,
DataItem data);
private:
void CleanUp();
private:
/* invariant: if a string 'name' is mapped to an id 'i' in 'name_to_id_'
it must be the position of a 'DataItem' object in 'data_'
and contained in at least one of the categories */
std::map<std::string, int> name_to_id_;
std::map<int, std::unordered_set<int>> categories_;
std::vector<DataItem> data_;
};
void MyContainer::CleanUp()
{
// Do your clean up here
}
void MyContainer::Insert(std::vector<std::string> names,
std::unordered_set<int> categories,
DataItem data)
{
bool failed = false;
/* ensure names, and categories are not empty and data is valid */
int id = data_.size();
for (auto it = names.begin(); it != names.end() && !failed; ++it) {
if (this->name_to_id_.count(*it))
failed = true;
try {
this->name_to_id_.insert({ *it, id });
}
catch (...) {
failed = true;
}
}
for (auto it = categories.begin(); it != categories.end() && !failed; ++it) {
try {
this->categories_.at(*it).insert(id);
}
catch (...) {
failed = true;
}
}
try {
if (!failed)
this->data_.push_back(data);
}
catch (...) {
failed = true;
}
if (failed)
CleanUp();
}
关于c++ - 同时插入STL容器并具有强大的异常保证,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/62970606/