base/trace_event/trace_conversion_helper.h

// Copyright 2020 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_TRACE_EVENT_TRACE_CONVERSION_HELPER_H_
#define BASE_TRACE_EVENT_TRACE_CONVERSION_HELPER_H_

#include <sstream>
#include <string>

#include "base/strings/string_number_conversions.h"
#include "base/template_util.h"
#include "base/trace_event/traced_value.h"

namespace base {

namespace trace_event {

// Helpers for base::trace_event::ValueToString.
namespace internal {

// Return std::string representation given by |value|'s ostream operator<<.
template <typename ValueType>
std::string OstreamValueToString(const ValueType& value) {
  std::stringstream ss;
  ss << value;
  return ss.str();
}

// Use SFINAE to decide how to extract a string from the given parameter.

// Check if |value| can be used as a parameter of |base::NumberToString|. If
// std::string is not constructible from the returned value of
// |base::NumberToString| cause compilation error.
//
// |base::NumberToString| does not do locale specific formatting and should be
// faster than using |std::ostream::operator<<|.
template <typename ValueType>
decltype(base::NumberToString(std::declval<const ValueType>()), std::string())
ValueToStringHelper(base::internal::priority_tag<5>,
                    const ValueType& value,
                    std::string /* unused */) {
  return base::NumberToString(value);
}

// If there is |ValueType::ToString| whose return value can be used to construct
// |std::string|, use this. Else use other methods.
template <typename ValueType>
decltype(std::string(std::declval<const ValueType>().ToString()))
ValueToStringHelper(base::internal::priority_tag<4>,
                    const ValueType& value,
                    std::string /* unused */) {
  return value.ToString();
}

// If |std::ostream::operator<<| can be used, use it. Useful for |void*|.
template <typename ValueType>
decltype(
    std::declval<std::ostream>().operator<<(std::declval<const ValueType>()),
    std::string())
ValueToStringHelper(base::internal::priority_tag<3>,
                    const ValueType& value,
                    std::string /* unused */) {
  return OstreamValueToString(value);
}

// Use |ValueType::operator<<| if applicable.
template <typename ValueType>
decltype(operator<<(std::declval<std::ostream&>(),
                    std::declval<const ValueType&>()),
         std::string())
ValueToStringHelper(base::internal::priority_tag<2>,
                    const ValueType& value,
                    std::string /* unused */) {
  return OstreamValueToString(value);
}

// If there is |ValueType::data| whose return value can be used to construct
// |std::string|, use it.
template <typename ValueType>
decltype(std::string(std::declval<const ValueType>().data()))
ValueToStringHelper(base::internal::priority_tag<1>,
                    const ValueType& value,
                    std::string /* unused */) {
  return value.data();
}

// Fallback returns the |fallback_value|. Needs to have |ValueToStringPriority|
// with the highest number (to be called last).
template <typename ValueType>
std::string ValueToStringHelper(base::internal::priority_tag<0>,
                                const ValueType& /* unused */,
                                std::string fallback_value) {
  return fallback_value;
}

/*********************************************
********* SetTracedValueArg methods. *********
*********************************************/

// base::internal::priority_tag parameter is there to define ordering in which
// the following methods will be considered. Note that for instance |bool| type
// is also |std::is_integral|, so we need to test |bool| before testing for
// integral.
template <typename T>
typename std::enable_if<std::is_same<T, bool>::value>::type
SetTracedValueArgHelper(base::internal::priority_tag<6>,
                        TracedValue* traced_value,
                        const char* name,
                        const T& value) {
  traced_value->SetBoolean(name, value);
}

// std::is_integral<bool>::value == true
// This needs to be considered only when T is not bool (has higher
// base::internal::priority_tag).
template <typename T>
typename std::enable_if<std::is_integral<T>::value>::type
SetTracedValueArgHelper(base::internal::priority_tag<5>,
                        TracedValue* traced_value,
                        const char* name,
                        const T& value) {
  // Avoid loss of precision.
  if (sizeof(int) < sizeof(value)) {
    // TODO(crbug.com/1111787): Add 64-bit support to TracedValue.
    traced_value->SetString(name, base::NumberToString(value));
  } else {
    traced_value->SetInteger(name, value);
  }
}

// Any floating point type is converted to double.
template <typename T>
typename std::enable_if<std::is_floating_point<T>::value>::type
SetTracedValueArgHelper(base::internal::priority_tag<4>,
                        TracedValue* traced_value,
                        const char* name,
                        const T& value) {
  traced_value->SetDouble(name, static_cast<double>(value));
}

// |void*| is traced natively.
template <typename T>
typename std::enable_if<std::is_same<T, void*>::value>::type
SetTracedValueArgHelper(base::internal::priority_tag<3>,
                        TracedValue* traced_value,
                        const char* name,
                        const T& value) {
  traced_value->SetPointer(name, value);
}

// |const char*| is traced natively.
template <typename T>
typename std::enable_if<std::is_same<T, const char*>::value>::type
SetTracedValueArgHelper(base::internal::priority_tag<2>,
                        TracedValue* traced_value,
                        const char* name,
                        const T& value) {
  traced_value->SetString(name, value);
}

// If an instance of |base::StringPiece| can be constructed from an instance of
// |T| trace |value| as a string.
template <typename T>
decltype(base::StringPiece(std::declval<const T>()), void())
SetTracedValueArgHelper(base::internal::priority_tag<1>,
                        TracedValue* traced_value,
                        const char* name,
                        const T& value) {
  traced_value->SetString(name, value);
}

// Fallback.
template <typename T>
void SetTracedValueArgHelper(base::internal::priority_tag<0>,
                             TracedValue* traced_value,
                             const char* name,
                             const T& /* unused */) {
  // TODO(crbug.com/1111787): Add fallback to |ValueToString|. Crashes on
  // operator<< have been seen with it.
  traced_value->SetString(name, "<value>");
}

}  // namespace internal

// The function to be used.
template <typename ValueType>
std::string ValueToString(const ValueType& value,
                          std::string fallback_value = "<value>") {
  return internal::ValueToStringHelper(base::internal::priority_tag<5>(), value,
                                       std::move(fallback_value));
}

// Method to trace |value| into the given |traced_value|. Support types where
// there is |TracedValue::SetT| natively.
//
// TODO(crbug.com/1111787): Add support for:
//   absl::optional
//   AsValueInto (T& and T*)
//   array and map types
//   fallback to ValueToString
template <typename ValueType>
void SetTracedValueArg(TracedValue* traced_value,
                       const char* name,
                       const ValueType& value) {
  internal::SetTracedValueArgHelper(base::internal::priority_tag<6>(),
                                    traced_value, name, value);
}

// Parameter pack support: do nothing for an empty parameter pack.
//
// Inline this to avoid linker duplicate symbol error.
inline void SetTracedValueArg(TracedValue* traced_value, const char* name) {}

// Parameter pack support. All of the packed parameters are traced under the
// same name. Serves to trace a parameter pack, all parameters having the same
// name (of the parameter pack) is desired.
//
// Example use when |args| is a parameter pack:
//   SetTracedValueArg(traced_value, name, args...);
template <typename ValueType, typename... ValueTypes>
void SetTracedValueArg(TracedValue* traced_value,
                       const char* name,
                       const ValueType& value,
                       const ValueTypes&... args) {
  SetTracedValueArg(traced_value, name, value);
  // Trace the rest from the parameter pack.
  SetTracedValueArg(traced_value, name, args...);
}

}  // namespace trace_event
}  // namespace base

#endif  // BASE_TRACE_EVENT_TRACE_CONVERSION_HELPER_H_