I’m changing a generation template of OpenAPI generator to help solving an issue affecting one user.

I’m stumbling upon what its generation causes:
A parameterToString(...) method offers many prototypes, and is used without troubles when its argument has a primitive type.
But it causes a compilation error for its templated const std::shared_ptr<T>& value or const boost::optional<const std::shared_ptr<T>>& value ones:

The method is declared in the ApiClient.h file generated:

/*
 * ApiClient.h
 * This is an API client responsible for stating the HTTP calls
 */
// [...]
class  ApiClient
{
public:
// [...]

    static utility::string_t parameterToString(utility::string_t value);
    static utility::string_t parameterToString(int32_t value);
    static utility::string_t parameterToString(int64_t value);
    static utility::string_t parameterToString(float value);
    static utility::string_t parameterToString(double value);
    static utility::string_t parameterToString(const utility::datetime &value);
    static utility::string_t parameterToString(bool value);
    template<class T>
    static utility::string_t parameterToString(const std::vector<T>& value);

    template<class T>
    utility::string_t parameterToString(const boost::optional<const std::shared_ptr<T>>& value);

    template<class T>
    utility::string_t parameterToString(const boost::optional<T>& value);

    template<class T>
    static utility::string_t parameterToString(const std::shared_ptr<T>& value);

// [...]
};

template<class T>
utility::string_t ApiClient::parameterToString(const std::vector<T>& value)
{
    utility::stringstream_t ss;

    for( size_t i = 0; i < value.size(); i++)
    {
        if( i > 0) ss << utility::conversions::to_string_t(", ");
        ss << ApiClient::parameterToString(value[i]);
    }

    return ss.str();
}

template<class T>
utility::string_t ApiClient::parameterToString(const boost::optional<T>& value)
{
    if (value.has_value()) {
        return parameterToString(*value.get());
    }
    else {
        return "** nullptr (A) **";
    }
}

template<class T>
utility::string_t ApiClient::parameterToString(const boost::optional<const std::shared_ptr<T>>& value)
{
    if (value.has_value()) {
        return parameterToString(*value.get());
    }
    else {
        return "** nullptr (B) **";
    }
}

template<class T>
utility::string_t ApiClient::parameterToString(const std::shared_ptr<T>& value)
{
    return parameterToString(*value.get());
}
}

And its attempt to be used fails here, for example:

pplx::task<std::shared_ptr<VideoChapters>> VideoChaptersApi::getVideoChapters(
   std::shared_ptr<_api_v1_videos_ownership__id__accept_post_id_parameter> id, 
   boost::optional<utility::string_t> xPeertubeVideoPassword) const
{
    // [...]

    boost::replace_all(localVarPath, 
      utility::conversions::to_string_t("{") + utility::conversions::to_string_t("id") + 
      utility::conversions::to_string_t("}"), 
      web::uri::encode_uri(ApiClient::parameterToString(id)));

    // [...]
}

where _api_v1_videos_ownership__id__accept_post_id_parameter is a class that has nothing special. it fails with the messages:

ApiClient.h: In instantiation of 
‘static utility::string_t org::openapitools::client::api::ApiClient::parameterToString(const std::shared_ptr<_Tp>&) 
[with T = org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter; utility::string_t = std::__cxx11::basic_string<char>]’:
LiveVideosApi.cpp:245:210:   required from here

ApiClient.h:105:29: error: no matching function for call to ‘org::openapitools::client::api::ApiClient::parameterToString(std::__shared_ptr<org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter, __gnu_cxx::_S_atomic>::element_type&)’
  105 |     return parameterToString(*value.get());
      |            ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~

ApiClient.h:89:19: note: candidate: ‘template<class T> static utility::string_t org::openapitools::client::api::ApiClient::parameterToString(const std::vector<_Type>&)’
   89 | utility::string_t ApiClient::parameterToString(const std::vector<T>& value)
      |                   ^~~~~~~~~
ApiClient.h:89:19: note:   template argument deduction/substitution failed:
ApiClient.h:105:29: note:   ‘std::__shared_ptr<org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter, __gnu_cxx::_S_atomic>::element_type’ {aka ‘org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter’} is not derived from ‘const std::vector<_Type>’
  105 |     return parameterToString(*value.get());
      |            ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~

ApiClient.h:103:19: note: candidate: ‘template<class T> static utility::string_t org::openapitools::client::api::ApiClient::parameterToString(const std::shared_ptr<_Tp>&)’
  103 | utility::string_t ApiClient::parameterToString(const std::shared_ptr<T>& value)
      |                   ^~~~~~~~~
ApiClient.h:103:19: note:   template argument deduction/substitution failed:
ApiClient.h:105:29: note:   ‘std::__shared_ptr<org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter, __gnu_cxx::_S_atomic>::element_type’ {aka ‘org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter’} is not derived from ‘const std::shared_ptr<_Tp>’
  105 |     return parameterToString(*value.get());
      |            ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~

ApiClient.h:59:30: note: candidate: ‘static utility::string_t org::openapitools::client::api::ApiClient::parameterToString(utility::string_t)’
   59 |     static utility::string_t parameterToString(utility::string_t value);
      |                              ^~~~~~~~~~~~~~~~~
ApiClient.h:59:66: note:   no known conversion for argument 1 from ‘std::__shared_ptr<org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter, __gnu_cxx::_S_atomic>::element_type’ {aka ‘org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter’} to ‘utility::string_t’ {aka ‘std::__cxx11::basic_string<char>’}
   59 |     static utility::string_t parameterToString(utility::string_t value);
      |                                                ~~~~~~~~~~~~~~~~~~^~~~~

ApiClient.h:60:30: note: candidate: ‘static utility::string_t org::openapitools::client::api::ApiClient::parameterToString(int32_t)’
   60 |     static utility::string_t parameterToString(int32_t value);
      |                              ^~~~~~~~~~~~~~~~~
ApiClient.h:60:56: note:   no known conversion for argument 1 from ‘std::__shared_ptr<org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter, __gnu_cxx::_S_atomic>::element_type’ {aka ‘org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter’} to ‘int32_t’ {aka ‘int’}
   60 |     static utility::string_t parameterToString(int32_t value);
      |                                                ~~~~~~~~^~~~~

ApiClient.h:61:30: note: candidate: ‘static utility::string_t org::openapitools::client::api::ApiClient::parameterToString(int64_t)’
   61 |     static utility::string_t parameterToString(int64_t value);
      |                              ^~~~~~~~~~~~~~~~~

ApiClient.h:61:56: note:   no known conversion for argument 1 from ‘std::__shared_ptr<org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter, __gnu_cxx::_S_atomic>::element_type’ {aka ‘org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter’} to ‘int64_t’ {aka ‘long int’}
   61 |     static utility::string_t parameterToString(int64_t value);
      |                                                ~~~~~~~~^~~~~

ApiClient.h:62:30: note: candidate: ‘static utility::string_t org::openapitools::client::api::ApiClient::parameterToString(float)’
   62 |     static utility::string_t parameterToString(float value);
      |                              ^~~~~~~~~~~~~~~~~

ApiClient.h:62:54: note:   no known conversion for argument 1 from ‘std::__shared_ptr<org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter, __gnu_cxx::_S_atomic>::element_type’ {aka ‘org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter’} to ‘float’
   62 |     static utility::string_t parameterToString(float value);
      |                                                ~~~~~~^~~~~

ApiClient.h:63:30: note: candidate: ‘static utility::string_t org::openapitools::client::api::ApiClient::parameterToString(double)’
   63 |     static utility::string_t parameterToString(double value);
      |                              ^~~~~~~~~~~~~~~~~

ApiClient.h:63:55: note:   no known conversion for argument 1 from ‘std::__shared_ptr<org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter, __gnu_cxx::_S_atomic>::element_type’ {aka ‘org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter’} to ‘double’
   63 |     static utility::string_t parameterToString(double value);
      |                                                ~~~~~~~^~~~~

ApiClient.h:64:30: note: candidate: ‘static utility::string_t org::openapitools::client::api::ApiClient::parameterToString(const utility::datetime&)’
   64 |     static utility::string_t parameterToString(const utility::datetime &value);
      |                              ^~~~~~~~~~~~~~~~~
ApiClient.h:64:73: note:   no known conversion for argument 1 from ‘std::__shared_ptr<org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter, __gnu_cxx::_S_atomic>::element_type’ {aka ‘org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter’} to ‘const utility::datetime&’
   64 |     static utility::string_t parameterToString(const utility::datetime &value);
      |                                                ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~

ApiClient.h:65:30: note: candidate: ‘static utility::string_t org::openapitools::client::api::ApiClient::parameterToString(bool)’
   65 |     static utility::string_t parameterToString(bool value);
      |                              ^~~~~~~~~~~~~~~~~
ApiClient.h:65:53: note:   no known conversion for argument 1 from ‘std::__shared_ptr<org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter, __gnu_cxx::_S_atomic>::element_type’ {aka ‘org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter’} to ‘bool’
   65 |     static utility::string_t parameterToString(bool value);
      |                                                ~~~~~^~~~~

I don’t understand how to handle this refusal:

ApiClient.h:103:19: note: candidate: ‘template<class T> static utility::string_t org::openapitools::client::api::ApiClient::parameterToString(const std::shared_ptr<_Tp>&)’
  103 | utility::string_t ApiClient::parameterToString(const std::shared_ptr<T>& value)
      |                   ^~~~~~~~~
ApiClient.h:103:19: note:   template argument deduction/substitution failed:
ApiClient.h:105:29: note:   ‘std::__shared_ptr<org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter, __gnu_cxx::_S_atomic>::element_type’ {aka ‘org::openapitools::client::model::_api_v1_videos_ownership__id__accept_post_id_parameter’} is not derived from ‘const std::shared_ptr<_Tp>’
  105 |     return parameterToString(*value.get());
      |            ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~

According to the message I’m reading, I should have an additional prototype available?

template<class T>
static utility::string_t parameterToString(const T& value);

?

But it looks to me dangerous to declare such one, as some cases, like boost::optional<...> parameters requires a special handling (just started here):

template<class T>
utility::string_t ApiClient::parameterToString(const boost::optional<const std::shared_ptr<T>>& value)
{
    if (value.has_value()) {
        return parameterToString(*value.get());
    }
    else {
        return "** nullptr (B) **";
    }
}

So, I don’t think it could/should be summarized in a “T& only” method.

How should I correct the template defined in ApiClient.h so that parameters of kind const std::shared_ptr<T>& value or const boost::optional<const std::shared_ptr<T>>& value will be accepted?

Thanks!