问题
std::vector has the member function at() as a safe alternative to operator[], so that bound checking is applied and no dangling references are created:
void foo(std::vector<int> const&x)
{
const auto&a=x[0]; // What if x.empty()? Undefined behavior!
const auto&a=x.at(0); // Throws exception if x.empty().
}
However, std::unique_ptr lacks the corresponding functionality:
void foo(std::unique_ptr<int> const&x)
{
const auto&a=*x; // What if bool(x)==false? Undefined behavior!
}
It would be great, if std::unique_ptr had such a safe alternative, say member ref() (and cref()) which never returns a dangling reference, but rather throws an exception. Possible implementation:
template<typename T>
typename add_lvalue_reference<T>::type
unique_ptr<T>::ref() const noexcept(false)
{
if(bool(*this)==false)
throw run_time_error("trying to de-refrence null unique_ptr");
return this->operator*();
}
Is there any good reason why the standard doesn't provide this sort of thing?
回答1:
I suspect the real answer is simple, and the same one for lots of "Why isn't C++ like this?" questions:
No-one proposed it.
std::vector and std::unique_ptr are not designed by the same people, at the same time, and are not used in the same way, so don't necessarily follow the same design principles.
回答2:
unique_ptr was specifically designed as a lightweight pointer class with null-state detection (e.g. stated in optional in A proposal to add a utility class to represent optional objects (Revision 3))
That said, the capability you're asking is already in-place since operator* documentation states:
// may throw, e.g. if pointer defines a throwing operator*
typename std::add_lvalue_reference<T>::type operator*() const;
The pointer type is defined as
std::remove_reference<Deleter>::type::pointer if that type exists, otherwise T*
Therefore through your custom deleter you're able to perform any on-the-fly operation including null pointer checking and exception throwing
#include <iostream>
#include <memory>
struct Foo { // object to manage
Foo() { std::cout << "Foo ctor\n"; }
Foo(const Foo&) { std::cout << "Foo copy ctor\n"; }
Foo(Foo&&) { std::cout << "Foo move ctor\n"; }
~Foo() { std::cout << "~Foo dtor\n"; }
};
struct Exception {};
struct InternalPtr {
Foo *ptr = nullptr;
InternalPtr(Foo *p) : ptr(p) {}
InternalPtr() = default;
Foo& operator*() const {
std::cout << "Checking for a null pointer.." << std::endl;
if(ptr == nullptr)
throw Exception();
return *ptr;
}
bool operator != (Foo *p) {
if(p != ptr)
return false;
else
return true;
}
void cleanup() {
if(ptr != nullptr)
delete ptr;
}
};
struct D { // deleter
using pointer = InternalPtr;
D() {};
D(const D&) { std::cout << "D copy ctor\n"; }
D(D&) { std::cout << "D non-const copy ctor\n";}
D(D&&) { std::cout << "D move ctor \n"; }
void operator()(InternalPtr& p) const {
std::cout << "D is deleting a Foo\n";
p.cleanup();
};
};
int main()
{
std::unique_ptr<Foo, D> up(nullptr, D()); // deleter is moved
try {
auto& e = *up;
} catch(Exception&) {
std::cout << "null pointer exception detected" << std::endl;
}
}
Live Example
For completeness' sake I'll post two additional alternatives/workarounds:
Pointer checking for a
unique_ptrviaoperator bool#include <iostream> #include <memory> int main() { std::unique_ptr<int> ptr(new int(42)); if (ptr) std::cout << "before reset, ptr is: " << *ptr << '\n'; ptr.reset(); if (ptr) std::cout << "after reset, ptr is: " << *ptr << '\n'; }(This would probably be the clanest way to deal with the issue)
An alternative solution, although messier, is to use a wrapper type which takes care of the exception handling
回答3:
I can't say, why the committee decided not to add a safe dereferenciation method - the answer is probably "because it wasn't proposed" or "because a raw pointer hasn't one either". But it is trivial to write a free function template on your own that takes any pointer as an argument, compares it against nullptr and then either throws an excepion or returns a reference to the pointed to object.
If you don't delete it via a pointer to base class, it should be even possible to derive publicly from a unique_ptr and just add such a member function.
Keep in mind however that using such a checked method everywhere might incur a significant performance hit (same as at). Usualy you want to validate your parameters at most once, for which a single if statement at the beginning is much better suited.
There is also the school that says you should not throw exceptions in response to programming errors. Maybe the peopke in charge of designing unique_ptr belonged to this school, while the people designing vector(which is much much older) didn't.
回答4:
One of the main goals of a smart pointer API design is to be a drop-in replacement with added value, no gotchas or side effects, and close to zero overhead. if (ptr) ptr->... is how safe access to bare pointer is usually done, the same syntax works nicely with smart pointers thus requiring no code change when one is replaced with the other.
An additional check for validity (say, to throw an exception) put inside a pointer would interfere with branch predictor and thus may have a knock-on effect on the performance, which may not be considered a zero cost drop-in replacement anymore.
回答5:
You do have
operator bool()
Example from: cplusplusreference
// example of unique_ptr::operator bool
#include <iostream>
#include <memory>
int main () {
std::unique_ptr<int> foo;
std::unique_ptr<int> bar (new int(12));
if (foo) std::cout << "foo points to " << *foo << '\n';
else std::cout << "foo is empty\n";
if (bar) std::cout << "bar points to " << *bar << '\n';
else std::cout << "bar is empty\n";
return 0;
}
unique_ptr is a simple wrapper to a raw pointer, no need to throw an exception when you can just check a boolean condition easily.
Edit: Apparently operator* can throw.
Exceptions 1) may throw, e.g. if pointer defines a throwing operator*
Maybe someone could shed some lights on hot to define a throwing operator*
回答6:
Following from the suggestion of MikeMB, here is a possible implementation of a free function for dereferencing pointers and unique_ptrs alike.
template<typename T>
inline T& dereference(T* ptr) noexcept(false)
{
if(!ptr) throw std::runtime_error("attempt to dereference a nullptr");
return *ptr;
}
template<typename T>
inline T& dereference(std::unique_ptr<T> const& ptr) noexcept(false)
{
if(!ptr) throw std::runtime_error("attempt to dereference an empty unique_ptr)");
return *ptr;
}
来源：https://stackoverflow.com/questions/32067213/why-is-there-no-safe-alternative-to-unique-ptroperator