Learning to read C++ Compiler Errors: Failing to Create a shared_ptr -- Raymond Chen

The trick to understanding C++ compiler error messages is to focus on two things. First, look at the beginning of the error message, which tells you what went wrong at a very low level. Then skip over the intermediate errors that follow the chain of calls until you end up at the line of code that you wrote. That original line of code is the one that is leading the compiler to a bad place. After that, you sometimes get supplemental information that helps you understand the low-level error better.

Learning to read C++ Compiler Errors: Failing to Create a shared_ptr

by Raymond Chen

From the article:

Consider the following erroneous code:

#include <memory>
#include <string>

struct WidgetOptions
{
    // imagine there is interesting stuff here
};

struct Widget
{
    Widget(WidgetOptions const* options);

    // imagine there is other interesting stuff here
};

void oops()
{
    WidgetOptions options;

    // The next line fails to compile
    std::shared_ptr<Widget> widget =
        std::make_shared<Widget>(options);
}

Here comes the error explosion.

// gcc
In file included from bits/stl_tempbuf.h:61,
                 from memory:66,
                 from sample.cpp:1:
bits/stl_construct.h: In instantiation of 'void std::_Construct(_Tp*, _Args&& ...) [with _Tp = Widget; _Args = {WidgetOptions&}]':
bits/alloc_traits.h:657:19:   required from 'static void std::allocator_traits<std::allocator<void> >::construct(allocator_type&, _Up*, _Args&& ...) [with _Up = Widget; _Args = {WidgetOptions&}; allocator_type = std::allocator<void>]'
  657 |         { std::_Construct(__p, std::forward<_Args>(__args)...); }
      |           ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
bits/shared_ptr_base.h:607:39:   required from 'std::_Sp_counted_ptr_inplace<_Tp, _Alloc, _Lp>::_Sp_counted_ptr_inplace(_Alloc, _Args&& ...) [with _Args = {WidgetOptions&}; _Tp = Widget; _Alloc = std::allocator<void>; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]'
  607 |           allocator_traits<_Alloc>::construct(__a, _M_ptr(),
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
  608 |               std::forward<_Args>(__args)...); // might throw
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
bits/shared_ptr_base.h:969:16:   required from 'std::__shared_count<_Lp>::__shared_count(_Tp*&, std::_Sp_alloc_shared_tag<_Alloc>, _Args&& ...) [with _Tp = Widget; _Alloc = std::allocator<void>; _Args = {WidgetOptions&}; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]'
  969 |           auto __pi = ::new (__mem)
      |                       ^~~~~~~~~~~~~
  970 |             _Sp_cp_type(__a._M_a, std::forward<_Args>(__args)...);
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
bits/shared_ptr_base.h:1713:14:   required from 'std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = std::allocator<void>; _Args = {WidgetOptions&}; _Tp = Widget; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]'
 1713 |         : _M_ptr(), _M_refcount(_M_ptr, __tag, std::forward<_Args>(__args)...)
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
bits/shared_ptr.h:463:59:   required from 'std::shared_ptr<_Tp>::shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = std::allocator<void>; _Args = {WidgetOptions&}; _Tp = Widget]'
  463 |         : __shared_ptr<_Tp>(__tag, std::forward<_Args>(__args)...)
      |                                                                  ^
bits/shared_ptr.h:1007:14:   required from 'std::shared_ptr<typename std::enable_if<(! std::is_array<_Tp>::value), _Tp>::type> std::make_shared(_Args&& ...) [with _Tp = Widget; _Args = {WidgetOptions&}; typename enable_if<(! is_array<_Tp>::value), _Tp>::type = Widget]'
 1007 |       return shared_ptr<_Tp>(_Sp_alloc_shared_tag<_Alloc>{__a},
      |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1008 |                              std::forward<_Args>(__args)...);
      |                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
sample.cpp:22:33:   required from here
   22 |         std::make_shared<Widget>(options);
      |         ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~
bits/stl_construct.h:119:7: error: no matching function for call to 'Widget::Widget(WidgetOptions&)'
  119 |       ::new((void*)__p) _Tp(std::forward<_Args>(__args)...);
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
sample.cpp:11:5: note: candidate: 'Widget::Widget(const WidgetOptions*)'
   11 |     Widget(WidgetOptions const* options);
      |     ^~~~~~
sample.cpp:11:33: note:   no known conversion for argument 1 from 'WidgetOptions' to 'const WidgetOptions*'
   11 |     Widget(WidgetOptions const* options);
      |            ~~~~~~~~~~~~~~~~~~~~~^~~~~~~
sample.cpp:9:8: note: candidate: 'constexpr Widget::Widget(const Widget&)'
    9 | struct Widget
      |        ^~~~~~
sample.cpp:9:8: note:   no known conversion for argument 1 from 'WidgetOptions' to 'const Widget&'
sample.cpp:9:8: note: candidate: 'constexpr Widget::Widget(Widget&&)'
sample.cpp:9:8: note:   no known conversion for argument 1 from 'WidgetOptions' to 'Widget&&'
Compiler returned: 1

 

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