by Emery Berger, Bryce Adelstein Lelbach, and Nicolas van Kempen
("See why")
Explains and suggest fixes for C/C /Rust compiler error messages.
python3 -m pip install cwhy
NOTE: To use cwhy, you must first set up an OpenAI API key. If you
already have an API key, you can set it as an environment variable
called OPENAI_API_KEY. If you do not have one yet,
you can get a key here: https://openai.com/api/.
export OPENAI_API_KEY=<your-api-key>
Just pipe your compiler's output to cwhy. cwhy will by default provide an explanation. If you'd like a suggested fix, add --fix.
e.g.,
% clang -g mycode.cpp |& cwhy # explanation only
% clang -g mycode.cpp |& cwhy --fix # to see a suggested fix
Expand to see the original (pretty obscure) error message:
In file included from test/test.cpp:1:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c /v1/unordered_set:391:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c /v1/__functional/is_transparent.h:14:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c /v1/type_traits:1838:38: error: implicit instantiation of undefined template 'std::hash<Q>'
: public integral_constant<bool, __is_empty(_Tp)> {};
^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c /v1/__memory/compressed_pair.h:34:15: note: in instantiation of template class 'std::is_empty<std::hash<Q> >' requested here
is_empty<_Tp>::value && !__libcpp_is_final<_Tp>::value>
^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c /v1/__memory/compressed_pair.h:110:35: note: in instantiation of default argument for '__compressed_pair_elem<std::hash<Q>, 1>' required here
private __compressed_pair_elem<_T2, 1> {
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c /v1/__hash_table:961:59: note: in instantiation of template class 'std::__compressed_pair<unsigned long, std::hash<Q> >' requested here
__compressed_pair<size_type, hasher> __p2_;
^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c /v1/unordered_set:428:13: note: in instantiation of template class 'std::__hash_table<Q, std::hash<Q>, std::equal_to<Q>, std::allocator<Q> >' requested here
__table __table_;
^
test/test.cpp:3:30: note: in instantiation of template class 'std::unordered_set<Q>' requested here
static std::unordered_set<Q> set;
^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c /v1/__memory/shared_ptr.h:1710:50: note: template is declared here
template <class _Tp> struct _LIBCPP_TEMPLATE_VIS hash;
^
1 error generated.
And here's the English-language explanation from cwhy:
% clang -g test/test.cpp |& cwhy
The error is caused by an implicit instantiation of an undefined
template 'std::hash<Q>'. It seems that the code is trying to use an
unordered_set with a custom type 'Q', but the standard library does
not know how to hash this type. The code needs to provide a hash
function for type 'Q' or specialize the 'std::hash' template for this
type.
And a proposed fix:
% clang test/test.cpp |& cwhy --fix
The problem is that `std::hash<Q>` is not defined, so it cannot be
implicitly instantiated. To fix the issue, define a hash function for
the `Q` struct. Here's an example:
```
#include <unordered_set>
struct Q {};
namespace std {
template<>
struct hash<Q> {
std::size_t operator()(const Q& q) const {
return std::hash<const void*>{}(&q);
}
};
}
static std::unordered_set<Q> set;
```
This code explicitly specializes the `std::hash` template for type
`Q`, and provides a `std::size_t operator()` implementation for the
hash function. In this implementation, we use the `std::hash<const
void*>` hash function to compute the hash value of the address of the
`Q` object. This is a reasonable, albeit not very efficient, way to
compute a hash value for an object.
% rustc test/testme.rs |& cwhy
There are three issues: 1. There are two unused variables `x` in the
code. 2. The variable `x` is used after it has already been moved in
the call to `f(x)`, which takes ownership of `x`. 3. The function
`f(x)` takes ownership of `x`, which may not be necessary and could be
changed to borrow the value instead.