Xcode C ++ Compilation Error Expected Member Name or ';' after declaration specifications

I'm having problems with validation methods in the C ++ library (openNN), which I am trying to compile in Xcode. I will use an example of one of the methods, since I suspect that they are all caused by the same problem.

The header declaration where I get the error message:
Expected member name or ';' after ad qualifiers.

void check(void) const;

Function Definition:

void InverseSumSquaredError::check(void) const
{
    std::ostringstream buffer;

    // Neural network stuff

    if(!neural_network_pointer)
    {
       buffer << "OpenNN Exception: InverseSumSquaredError class.\n"
              << "void check(void) const method.\n"
              << "Pointer to neural network is NULL.\n";

       throw std::logic_error(buffer.str().c_str());      
    }

    const MultilayerPerceptron* multilayer_perceptron_pointer = neural_network_pointer->get_multilayer_perceptron_pointer();

    if(!multilayer_perceptron_pointer)
    {
       buffer << "OpenNN Exception: InverseSumSquaredError class.\n"
              << "void check(void) const method.\n"
              << "Pointer to multilayer perceptron is NULL.\n";

       throw std::logic_error(buffer.str().c_str());      
    }

    const unsigned int inputs_number = multilayer_perceptron_pointer->count_inputs_number();
    const unsigned int outputs_number = multilayer_perceptron_pointer->count_outputs_number();

    if(inputs_number == 0)
    {
       buffer << "OpenNN Exception: InverseSumSquaredError class.\n"
              << "void check(void) const method.\n"
              << "Number of inputs in multilayer perceptron object is zero.\n";

       throw std::logic_error(buffer.str().c_str());      
    }

    if(outputs_number == 0)
    {
       buffer << "OpenNN Exception: InverseSumSquaredError class.\n"
         << "void check(void) const method.\n"
         << "Number of outputs in multilayer perceptron object is zero.\n";

       throw std::logic_error(buffer.str().c_str());      
    }

    // Mathematical model stuff

    if(!mathematical_model_pointer)
    {
        buffer << "OpenNN Exception: InverseSumSquaredError class.\n"
               << "void check(void) const method.\n"
               << "Pointer to mathematical model is NULL.\n";

        throw std::logic_error(buffer.str().c_str());     
    }

    // Data set stuff 

    if(!data_set_pointer)
    {
       buffer << "OpenNN Exception: InverseSumSquaredError class.\n"
              << "void check(void) const method.\n"
              << "Pointer to data set is NULL.\n";

       throw std::logic_error(buffer.str().c_str());      
    }

    // Final solutions error stuff

}

void InverseSumSquaredError::check(void) const;
:

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