Automatically wrapping values for the duration of a statement - c++

Question

Is it possible to automatically wrap a value in a temporary whose lifetime extends across the entire statement?
Originally I hoped a solution or alternative to my problem would present itself while writing the details for the question, unfortunately that didn't happen, so...
I have an abstract base class Logger that provides a streaming-like interface for generating log statements. Given an instance logger of this class, I want the following to be possible:
logger << "Option " << variable << " is " << 42;
Unlike regular streams, which simply generate a string from all the components (4 components in the example above), I want to generate an instance of a class Statement that manages a linked list of all the statement's components. The entire statement is then passed via pure virtual method to a class derived from Logger, which can iterate over all the components of the statement and do whatever with them, including obtaining information about their type, retrieving their value, or converting them to a string.
The tricky bit: I want to do the above without dynamic memory allocations. This means that every component of the statement must be wrapped by a temporary type that links the components into a traversable list, within the scope of the statement!
I posted a working example on ideone, with one problem: every component needs to be wrapped by a function call in order to generate an instance of the temporary type. The log statement therefore ends up looking like this:
logger << wrap("Option ") << wrap(variable) << wrap(" is ") << wrap(42);
All my attempts to get rid of the wrap function (e.g., using an implicit converting constructor for the component), have failed thus far, therefore this question.
How can the components of the log statement automatically be wrapped in their component type (e.g., using a converting constructor for the component), without the need for an explicit call to a wrapping function?
Alternatively, I would appreciate suggestions for other ways that achieve the same effect, i.e., allowing iteration over the components of the log statement in a class derived from logger, without dynamic memory allocations.
Reference: Full code on ideone:
#include <iostream>
#include <sstream>
struct Statement;
struct Logger;
struct ComponentBase;
//------------------------------------------------------------------------------
struct ComponentBase {
mutable ComponentBase *next;
ComponentBase() : next(nullptr) { }
virtual std::string toString() = 0;
};
template <typename T>
struct Component : ComponentBase {
T value;
Component(T value) : value(value) { }
~Component() { }
virtual std::string toString() {
std::stringstream ss;
ss << value;
return ss.str();
}
};
struct ComponentIterator {
ComponentBase *ptr;
ComponentIterator(ComponentBase *ptr) : ptr(ptr) { }
ComponentBase &operator*() { return *ptr; }
void operator++() { ptr = ptr->next; }
bool operator!=(ComponentIterator &other) { return (ptr != other.ptr); }
};
//------------------------------------------------------------------------------
struct Statement {
Logger *logger;
ComponentBase *front;
ComponentBase *back;
ComponentIterator begin() { return front; }
ComponentIterator end() { return nullptr; }
template <typename T>
Statement(Logger &logger, Component<T> &component)
: logger(&logger), front(&component), back(&component) { }
~Statement();
template <typename T>
Statement &operator<<(Component<T> &&component) {
back->next = &component;
back = &component;
return *this;
}
};
//------------------------------------------------------------------------------
struct Logger {
template <typename T>
Statement operator<<(Component<T> &&component) {
return {*this, component};
}
virtual void log(Statement &statement) = 0;
};
Statement::~Statement() {
logger->log(*this);
}
//------------------------------------------------------------------------------
template <typename T>
Component<T const &> wrap(T const &value) {
return value;
}
template <size_t N>
Component<char const *> wrap(char const (&value)[N]) {
return value;
}
//------------------------------------------------------------------------------
struct MyLogger : public Logger {
virtual void log(Statement &statement) override {
for(auto &&component : statement) {
std::cout << component.toString();
}
std::cout << std::endl;
}
};
int main() {
std::string variable = "string";
MyLogger logger;
logger << wrap("Option ") << wrap(variable) << wrap(" is ") << wrap(42);
}

Answer 1

I have some crazy but working solution.
Having component implemented like this you will get rid of templates all over your code:
struct Component
{
mutable Component *next;
typedef std::function<std::string()> ToStringFunction;
ToStringFunction toString; // <-- 1
template<typename T>
Component(const T& value)
: next(nullptr),
toString(nullptr)
{
toString = [&value](){
std::stringstream ss;
ss << value;
return ss.str();
};
}
};
Where (1) is the unction that knows what to do. This member std::function is a space for optimization.
And the rest of the code should look like:
struct ComponentIterator {
Component *ptr;
ComponentIterator(Component *ptr) : ptr(ptr) { }
Component &operator*() { return *ptr; }
void operator++() { ptr = ptr->next; }
bool operator!=(ComponentIterator &other) { return (ptr != other.ptr); }
};
//------------------------------------------------------------------------------
struct Statement {
Logger *logger;
Component *front;
Component *back;
ComponentIterator begin() { return front; }
ComponentIterator end() { return nullptr; }
Statement(Logger &logger, Component &component)
: logger(&logger), front(&component), back(&component) { }
~Statement();
Statement &operator<<(Component &&component) {
back->next = &component;
back = &component;
return *this;
}
};
//------------------------------------------------------------------------------
struct Logger {
Statement operator<<(Component &&component) {
return{ *this, component };
}
virtual void log(Statement &statement) = 0;
};
Statement::~Statement() {
logger->log(*this);
}
//------------------------------------------------------------------------------
struct MyLogger : public Logger {
virtual void log(Statement &statement) override {
for (auto &&component : statement) {
std::cout << component.toString();
}
std::cout << std::endl;
}
};
int main() {
std::string variable = "string";
MyLogger logger;
//logger << wrap("Option ") << wrap(variable) << wrap(" is ") << wrap(42);
logger << 42;
logger << variable << " is " << 42;
logger << "Option " << variable << " is " << 42;
}
this will print:
42
string is 42
Option string is 42
UPD
as dyp advised here is alternative implementation of the Component structure without lambda:
struct Component
{
mutable Component *next;
void* value;
std::string toString(){
return _toString(this);
}
template<typename T>
Component(const T& inValue)
: next(nullptr),
value((void*)&inValue),
_toString(toStringHelper<T>)
{}
private:
typedef std::string(*ToStringFunction)(Component*);
ToStringFunction _toString;
template<typename T>
static std::string toStringHelper(Component* component)
{
const T& value = *(T*)component->value;
std::stringstream ss;
ss << value;
return ss.str();
}
};

Answer 2

I propose a solution tuple based:
template <class... Ts> class streamTuple;
struct Logger {
template <typename T>
streamTuple<T> operator<<(const T& t);
template <typename Tuple, std::size_t ... Is>
void dispatch(const Tuple& tup, std::index_sequence<Is...>)
{
int dummy[] = {0, (void(std::cout << std::get<Is>(tup) << " "), 0)...};
static_cast<void>(dummy); // Avoid unused variable warning
}
// Logger can take generic functor to have specific dispatch
// Or you may reuse your virtual method taking ComponentBase.
};
template <class... Ts> class streamTuple
{
public:
streamTuple(Logger* logger, const std::tuple<Ts...>& tup) :
logger(logger), tup(tup) {}
streamTuple(streamTuple&& rhs) : logger(rhs.logger), tup(std::move(rhs.tup))
{
rhs.logger = nullptr;
}
~streamTuple()
{
if (logger) {
logger->dispatch(tup, std::index_sequence_for<Ts...>());
}
}
template <typename T>
streamTuple<Ts..., const T&> operator << (const T& t) &&
{
auto* moveddLogger = logger;
logger = nullptr;
return {moveddLogger, std::tuple_cat(tup, std::tie(t))};
}
private:
Logger* logger;
std::tuple<Ts...> tup;
};
template <typename T>
streamTuple<T> Logger::operator<<(const T& t) {
return {this, t};
}
Demo
And usage:
int main() {
Logger log;
std::string variable = "string";
log << variable << 42 << "hello\n";
}