我目前正在使用https://casablanca.codeplex.com/上的 cpprest API 来创建一个简单的 RESTful API 项目。
这个流行的库使用 Microsoft 的 PPLX 框架来促进异步任务。
我目前遇到一个问题，当函数返回类型对象pplx::task<SomeObj>并且SomeObj没有默认构造函数时，代码无法编译。我的理解是这段代码应该是有效的，因为不应该创建默认对象。
我的代码中有错误还是这是一个错误？
示例代码：
#include <vector>
#include <string>
#include <cpprest/http_client.h>
class SomeSubObj {
public:
int a,b,c;
SomeSubObj(int a, int b, int c): a(a), b(b), c(c){}
};
class SomeObj {
public:
std::vector<SomeSubObj> subObjs;
SomeObj(std::string json){
// Populate from JSON...
subObjs.push_back(SomeSubObj(1,1,1));
subObjs.push_back(SomeSubObj(2,2,2));
subObjs.push_back(SomeSubObj(3,3,3));
}
};
pplx::task<SomeObj> doSomething(){
return pplx::task<std::string>([]() {
// Make a call out and get some json...
return std::string("{...}");
}).then([](std::string x) {
// Return a transformed object
return SomeObj("...");
});
}
int main()
{
doSomething();
return 0;
}
我目前收到以下输出：
g++ -std=c++0x -Iboost_thread-mt -Iboost_atomic -Iboost_system -Icpprest -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/Bug.d" -MT"src/Bug.d" -o "src/Bug.o" "../src/Bug.cpp"
In file included from ../src/Bug.cpp:4:
In file included from /usr/local/include/cpprest/http_client.h:44:
/usr/local/include/pplx/pplxtasks.h:2404:9: error: call to implicitly-deleted default constructor of '_ResultHolder<SomeObj>'
_Task_impl(_CancellationTokenState * _Ct, scheduler_ptr _Scheduler_arg)
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:2096:15: note: in instantiation of member function 'pplx::details::_Task_impl<SomeObj>::_Task_impl' requested here
__second_(_VSTD::forward<_Args2>(get<_I2>(__second_args))...)
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:2359:15: note: in instantiation of function template specialization 'std::__1::__libcpp_compressed_pair_imp<std::__1::allocator<pplx::details::_Task_impl<SomeObj> >, pplx::details::_Task_impl<SomeObj>, 1>::__libcpp_compressed_pair_imp<std::__1::allocator<pplx::details::_Task_impl<SomeObj> > &, pplx::details::_CancellationTokenState *&, pplx::scheduler_ptr &, 0, 0, 1>' requested here
: base(__pc, _VSTD::move(__first_args), _VSTD::move(__second_args),
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:3697:16: note: in instantiation of function template specialization 'std::__1::__compressed_pair<std::__1::allocator<pplx::details::_Task_impl<SomeObj> >, pplx::details::_Task_impl<SomeObj> >::__compressed_pair<std::__1::allocator<pplx::details::_Task_impl<SomeObj> > &, pplx::details::_CancellationTokenState *&, pplx::scheduler_ptr &>' requested here
: __data_(piecewise_construct, _VSTD::forward_as_tuple(__a),
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:4271:26: note: in instantiation of function template specialization 'std::__1::__shared_ptr_emplace<pplx::details::_Task_impl<SomeObj>, std::__1::allocator<pplx::details::_Task_impl<SomeObj> > >::__shared_ptr_emplace<pplx::details::_CancellationTokenState *&, pplx::scheduler_ptr &>' requested here
::new(__hold2.get()) _CntrlBlk(__a2, _VSTD::forward<_Args>(__args)...);
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:4630:29: note: in instantiation of function template specialization 'std::__1::shared_ptr<pplx::details::_Task_impl<SomeObj> >::make_shared<pplx::details::_CancellationTokenState *&, pplx::scheduler_ptr &>' requested here
return shared_ptr<_Tp>::make_shared(_VSTD::forward<_Args>(__args)...);
^
/usr/local/include/pplx/pplxtasks.h:1429:103: note: (skipping 1 context in backtrace; use -ftemplate-backtrace-limit=0 to see all)
static _Type _Make(_CancellationTokenState * _Ct, scheduler_ptr _Scheduler_arg) { return std::make_shared<_Task_impl<_ReturnType>>(_Ct, _Scheduler_arg); }
^
/usr/local/include/pplx/pplxtasks.h:3623:52: note: in instantiation of member function 'pplx::details::_Task_ptr<SomeObj>::_Make' requested here
_M_Impl = details::_Task_ptr<_ReturnType>::_Make(_Ct, _Scheduler);
^
/usr/local/include/pplx/pplxtasks.h:4138:27: note: in instantiation of member function 'pplx::task<SomeObj>::_CreateImpl' requested here
_ContinuationTask._CreateImpl(_PTokenState, _Scheduler);
^
/usr/local/include/pplx/pplxtasks.h:4101:16: note: in instantiation of function template specialization 'pplx::task<std::__1::basic_string<char> >::_ThenImpl<std::__1::basic_string<char>, <lambda at ../src/Bug.cpp:27:10> >' requested here
return _ThenImpl<_InternalReturnType, _Function>(_Func, _PTokenState, _TaskOptions.get_continuation_context(), _Scheduler, _CreationStack);
^
/usr/local/include/pplx/pplxtasks.h:3424:16: note: in instantiation of function template specialization 'pplx::task<std::__1::basic_string<char> >::_ThenImpl<std::__1::basic_string<char>, <lambda at ../src/Bug.cpp:27:10> >' requested here
return _ThenImpl<_ReturnType, _Function>(_Func, _TaskOptions);
^
../src/Bug.cpp:27:5: note: in instantiation of function template specialization 'pplx::task<std::__1::basic_string<char> >::then<<lambda at ../src/Bug.cpp:27:10> >' requested here
}).then([](std::string x) {
^
/usr/local/include/pplx/pplxtasks.h:844:15: note: default constructor of '_ResultHolder<SomeObj>' is implicitly deleted because field '_Result' has no default constructor
_Type _Result;
^
1 error generated.
make: *** [src/Bug.o] Error 1