问题
Suppose I have a function called loadData() which takes a container (to be filled with data) and a CSV file. I need the following overloads:
loadData(std::vector<double>& data, const std::string& file);loadData(QVector<double>& data, const std::string& file);loadData(std::vector<std::complex<double>>& data, const std::string& file);loadData(QVector<std::complex<double>>& data, const std::string& file);loadData(std::vector<std::vector<double>>& data, const std::string& file);loadData(QVector<QVector<double>>& data, const std::string& file);loadData(std::vector<std::vector<std::complex<double>>>& data, const std::string& file);loadData(QVector<QVector<std::complex<double>>>& data, const std::string& file);
QVector is Qt's vector-like class with a similar API, but taking only one template argument T (instead of two, like std::vector<T, A>).
The implementation for 1-4 is almost the same (it just calls 5-8, wrapping that single vector in another vector (of the same type!).
The implementation for 5-8 is the same (it's just calling the appropriate overload of the CSV parsing function).
I may also need to add float overloads, or QVector<std::vector<...>> / std::vector<QVector<...>> overloads.
Basically, it's a huge set of overloads that I'd like to generalize.
Is it possible to combine all of them into maybe 2 function templates (one for 1D containers, another for 2D containers)?
Thanks
回答1:
Is it possible to combine all of them into maybe 2 function templates (one for 1D containers, another for 2D containers)?
Yes... require a little work but is relatively simple.
Maybe there are simpler methods but I propose the create a custom type traits that verify if a template-template is std::vector or QVector
template <template <typename...> class>
struct isVector : public std::false_type
{ };
template <>
struct isVector<std::vector> : public std::true_type
{ };
template <>
struct isVector<QVector> : public std::true_type
{ };
Next another custom type traits to verify if a type is a floating point type of a std::complex<T> type
template <typename T>
struct isCFloating : public std::is_floating_point<T>
{ };
template <typename T>
struct isCFloating<std::complex<T>> : public std::true_type
{ };
Now you can write the vector-of-vector version (that intercept also mixed std::vector/QVector case) as follows
template <template <typename ...> class V1,
template <typename ...> class V2, typename T>
auto loadData (V1<V2<T>> & v, std::string fn)
-> std::enable_if_t< isVector<V1>::value
&& isVector<V2>::value
&& isCFloating<T>::value>
{
std::cout << "- vector of vector version, " << fn << std::endl;
}
and, after, the simple vector version (that wrap the first argument and call the vector-of-vector version) become
template <template <typename ...> class V, typename T>
auto loadData (V<T> & v, std::string fn)
-> std::enable_if_t<isVector<V>::value && isCFloating<T>::value>
{
std::cout << "- vector version, " << fn << std::endl;
V<V<T>> vv{1, v};
loadData(vv, fn);
}
I've prepared the following full working example but (sorry) I don't have a QT platform available at the moment so I have added a fake QVector
#include <vector>
#include <complex>
#include <iostream>
#include <type_traits>
// fake QVector
template <typename>
struct QVector
{
template <typename ... Ts>
QVector (Ts const & ...)
{ }
};
template <template <typename...> class>
struct isVector : public std::false_type
{ };
template <>
struct isVector<std::vector> : public std::true_type
{ };
template <>
struct isVector<QVector> : public std::true_type
{ };
template <typename T>
struct isCFloating : public std::is_floating_point<T>
{ };
template <typename T>
struct isCFloating<std::complex<T>> : public std::true_type
{ };
template <template <typename ...> class V1,
template <typename ...> class V2, typename T>
auto loadData (V1<V2<T>> & v, std::string fn)
-> std::enable_if_t< isVector<V1>::value
&& isVector<V2>::value
&& isCFloating<T>::value>
{
std::cout << "- vector of vector version, " << fn << std::endl;
}
template <template <typename ...> class V, typename T>
auto loadData (V<T> & v, std::string fn)
-> std::enable_if_t<isVector<V>::value && isCFloating<T>::value>
{
std::cout << "- vector version, " << fn << std::endl;
V<V<T>> vv{1, v};
loadData(vv, fn);
}
int main ()
{
std::vector<float> vf;
std::vector<std::complex<float>> vc;
std::vector<std::vector<double>> vvf;
std::vector<std::vector<std::complex<double>>> vvc;
QVector<long double> qf;
QVector<std::complex<long double>> qc;
QVector<QVector<float>> qqf;
QVector<QVector<std::complex<float>>> qqc;
loadData(vf, "case 1");
loadData(qf, "case 2");
loadData(vc, "case 3");
loadData(qc, "case 4");
loadData(vvf, "case 5");
loadData(qqf, "case 6");
loadData(vvc, "case 7");
loadData(qqc, "case 8");
// extra cases: mixing std::vector and QVector
std::vector<QVector<double>> vqf;
std::vector<QVector<std::complex<double>>> vqc;
QVector<std::vector<long double>> qvf;
QVector<std::vector<std::complex<long double>>> qvc;
loadData(vqf, "case 9");
loadData(vqc, "case 10");
loadData(qvf, "case 11");
loadData(qvc, "case 12");
}
回答2:
You can use templates and iterators based generic algorithms to reduce code duplication. For example:
enum class load_data_status
{
success
// something you need
};
template<typename I>
void loadData(const I& first,const I& last, std::ostream& file)
{
typedef typename std::iterator_traits<I>::value_type T;
// do load raw data
std::for_each(first, last, [file](const T& t) {
// do something
} );
}
template<typename I>
void loadComplexData(const I& first,const I& last, std::ostream& file)
{
typedef typename std::iterator_traits<I>::value_type C;
typedef typename C::value_type T;
// do load complex data
std::for_each(first, last, [file](const C& t) {
// do something
} );
}
template<typename T>
load_data_status loadData(const std::vector< std::vector<T> >& data, const std::string& file) {
std::ofstream f(file);
std::for_each(data.cbegin(), data.cend(), [f] (const std::vector<T>& v) {
loadData(v.cbegin(), v.cend(), f);
} );
return load_data_status::success;
}
template<typename T>
load_data_status loadData(const QVector< QVector<T> >& data, const std::string& file) {
std::ofstream f(file);
std::for_each(data.begin(), data.end(), [f] (const QVector<T>& v) {
loadData(v.begin(), v.end(), f);
} );
return load_data_status::success;
}
template<typename T>
load_data_status loadData(const std::vector< std::vector<std::complex<T> > >& data, const std::string& file) {
std::ofstream f(file);
std::for_each(data.cbegin(), data.cend(), [f] (const std::vector<std::complex<T> >& v) {
loadComplexData(v.cbegin(), v.cend(), f);
} );
return load_data_status::success;
}
template<typename T>
load_data_status loadData(const QVector< QVector< std::complex<T> > >& data, const std::string& file) {
std::ofstream f(file);
std::for_each(data.begin(), data.end(), [f] (const QVector< std::complex<T> >& v) {
loadComplexData(v.begin(), v.end(), f);
} );
return load_data_status::success;
}
回答3:
When writing generic code, pay particular attention to separating concerns.
There are two concerns in loadData :-
- getting objects from a file
- appending those objects to a container
So let's build a template for acting as the data source (which we can specialise based on the value type), and then write one overload per container type in terms of that.
First attempt:
#include <fstream>
#include <vector>
#include <iterator>
#include <algorithm>
#include <complex>
struct data_file_loader_base
{
data_file_loader_base(std::string const& path)
: input_stream_(path)
, items_(get_input<std::size_t>())
{
}
template<class T>
T get_input()
{
T val;
input_stream_ >> val;
return val;
}
std::size_t items() const
{
return items_;
}
std::ifstream& input_stream()
{
return input_stream_;
}
private:
std::ifstream input_stream_;
std::size_t items_;
};
// basic value loader
template<class Type>
struct data_file_loader
: data_file_loader_base
{
data_file_loader(std::string const& path)
: data_file_loader_base(path)
{}
std::istream_iterator<Type> begin()
{
return { input_stream() };
}
std::istream_iterator<Type> end()
{
return { };
}
};
// specialize for types that need custom input
// in this case, for a complex. But could easily be a vector etc.
template<class Type>
struct data_file_loader<std::complex<Type>>
: data_file_loader_base
{
data_file_loader(std::string const& path)
: data_file_loader_base(path)
{}
struct iterator
{
using value_type = std::complex<Type>;
using iterator_category = std::forward_iterator_tag;
using pointer = value_type*;
using reference = value_type&;
using difference_type = int;
iterator(data_file_loader& loader, bool eof = false)
: self_(loader)
, eof_(eof || check_eof())
{
}
bool operator==(iterator const& other) const
{
return eof_ && other.eof_;
}
bool operator!=(iterator const& other) const
{
return !(*this == other);
}
iterator& operator++() {
return *this;
}
value_type operator*() {
auto result = value_type { self_.get_input<Type>(), self_.get_input<Type>() };
eof_ = check_eof();
return result;
}
bool check_eof() const {
return !input_stream();
}
data_file_loader& self_;
bool eof_ = false;
};
iterator begin()
{
return { *this, false };
}
iterator end()
{
return { *this, true };
}
};
// one overload per container type
template<class Type>
void loadData(std::vector<Type>& target, std::string const& path)
{
auto loader = data_file_loader<Type>(path);
target.reserve(loader.items());
std::copy(std::begin(loader), std::end(loader), std::back_inserter(target));
}
// test
int main()
{
std::vector<int> vi;
loadData(vi, "foo.txt");
std::vector<std::complex<double>> vc;
loadData(vc, "foo_complex.txt");
}
来源：https://stackoverflow.com/questions/50231612/a-function-template-that-accepts-both-stdvector-and-qvector