[FEATURE] ways to save logs of search path [PSO]

[yezhengli-Mr9]

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#595
to save parameters in the search path, for example, in PSO.
Describe the solution you'd like
Making attempt of putting

mutable std::mutex file_mutex;

int tscall::write_to_file(std::string content) const {
    std::lock_guard<std::mutex> lock(file_mutex);  // Locking to prevent race condition
    std::ofstream file(filename, std::ios::app);   // Open in append mode
    if (file.is_open()) {
        file << content << std::endl;
    } else {
        std::cerr << "Error opening file!" << std::endl;
    }
}    

in

pagmo2/src/problems/rosenbrock.cpp

Lines 51 to 59 in ac7b6d2

	/// Fitness computation
	/**
	* Computes the fitness for this UDP.
	*
	* @param x the decision vector.
	*
	* @return the fitness of \p x.
	*/
	vector_double rosenbrock::fitness(const vector_double &x) const

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