我是c ++的新手,我试图了解这些代码是如何工作的,我能够理解includeguard的工作方式。以及如何使用嵌套名称空间gtl和tensorflow。
我想了解这段代码的工作原理
// This file provides utility functions for use with STL map-like data
// structures, such as std::map and hash_map. Some functions will also work with
// sets, such as ContainsKey().
#ifndef TENSORFLOW_LIB_GTL_MAP_UTIL_H_
#define TENSORFLOW_LIB_GTL_MAP_UTIL_H_
#include <stddef.h>
#include <iterator>
#include <memory>
#include <string>
#include <utility>
namespace tensorflow {
namespace gtl {
// Returns a pointer to the const value associated with the given key if it
// exists, or NULL otherwise.
template <class Collection>
const typename Collection::value_type::second_type* FindOrNull(
const Collection& collection,
const typename Collection::value_type::first_type& key) {
typename Collection::const_iterator it = collection.find(key);
if (it == collection.end()) {
return 0;
}
return &it->second;
}
// Same as above but returns a pointer to the non-const value.
template <class Collection>
typename Collection::value_type::second_type* FindOrNull(
Collection& collection, // NOLINT
const typename Collection::value_type::first_type& key) {
typename Collection::iterator it = collection.find(key);
if (it == collection.end()) {
return 0;
}
return &it->second;
}
// Returns the pointer value associated with the given key. If none is found,
// NULL is returned. The function is designed to be used with a map of keys to
// pointers.
//
// This function does not distinguish between a missing key and a key mapped
// to a NULL value.
template <class Collection>
typename Collection::value_type::second_type FindPtrOrNull(
const Collection& collection,
const typename Collection::value_type::first_type& key) {
typename Collection::const_iterator it = collection.find(key);
if (it == collection.end()) {
return typename Collection::value_type::second_type();
}
return it->second;
}
// Returns a const reference to the value associated with the given key if it
// exists, otherwise returns a const reference to the provided default value.
//
// WARNING: If a temporary object is passed as the default "value,"
// this function will return a reference to that temporary object,
// which will be destroyed at the end of the statement. A common
// example: if you have a map with string values, and you pass a char*
// as the default "value," either use the returned value immediately
// or store it in a string (not string&).
template <class Collection>
const typename Collection::value_type::second_type& FindWithDefault(
const Collection& collection,
const typename Collection::value_type::first_type& key,
const typename Collection::value_type::second_type& value) {
typename Collection::const_iterator it = collection.find(key);
if (it == collection.end()) {
return value;
}
return it->second;
}
// Inserts the given key and value into the given collection if and only if the
// given key did NOT already exist in the collection. If the key previously
// existed in the collection, the value is not changed. Returns true if the
// key-value pair was inserted; returns false if the key was already present.
template <class Collection>
bool InsertIfNotPresent(Collection* const collection,
const typename Collection::value_type& vt) {
return collection->insert(vt).second;
}
// Same as above except the key and value are passed separately.
template <class Collection>
bool InsertIfNotPresent(
Collection* const collection,
const typename Collection::value_type::first_type& key,
const typename Collection::value_type::second_type& value) {
return InsertIfNotPresent(collection,
typename Collection::value_type(key, value));
}
// Looks up a given key and value pair in a collection and inserts the key-value
// pair if it's not already present. Returns a reference to the value associated
// with the key.
template <class Collection>
typename Collection::value_type::second_type& LookupOrInsert(
Collection* const collection, const typename Collection::value_type& vt) {
return collection->insert(vt).first->second;
}
// Same as above except the key-value are passed separately.
template <class Collection>
typename Collection::value_type::second_type& LookupOrInsert(
Collection* const collection,
const typename Collection::value_type::first_type& key,
const typename Collection::value_type::second_type& value) {
return LookupOrInsert(collection,
typename Collection::value_type(key, value));
}
} // namespace gtl
} // namespace tensorflow
#endif // TENSORFLOW_LIB_GTL_MAP_UTIL_H_
int main(int argc, char const *argv[])
{
}
我真的很想了解这部分的工作原理。
// exists, or NULL otherwise.
template <class Collection>
const typename Collection::value_type::second_type* FindOrNull(
const Collection& collection,
const typename Collection::value_type::first_type& key) {
typename Collection::const_iterator it = collection.find(key);
if (it == collection.end()) {
return 0;
}
return &it->second;
}
我已经使用过c ++ map,但是我仍然不明白这一点。
我从未见过如何在此处使用关键字模板。
我非常努力地了解它是如何工作的,但是也许我的基本知识尚不清楚,所以我无法理解这一点。能否请您举例说明这个用法和简短说明?
最佳答案
让我们分解一下(我已经用int
,std::string
和const_iterator
(可能不是实际类型)替换了一些复杂的类型):
template <class Collection>
const int FindOrNull(
const Collection& collection,
const std::string& key) {
const_iterator it = collection.find(key);
if (it == collection.end())
return 0;
return &it->second;
}
因此,您可以开始使用-一个简单的函数,它带有两个参数并返回一个整数。该
template <class Collection>
表示FindOrNull
是模板函数,除非具有某些必需的方法/属性(例如Collection
),否则它可以采用(cc)类型的参数。模板函数的一个简单示例:
template <typename T, typename Y>
auto add(const T &a, const Y &b) -> decltype(a + b) {
return a + b;
}
此函数添加了两个...东西:两个整数,
Collection::find
s,double
s,float
s,无论可以加在一起的什么。关于c++ - 从Tensorflow C++ API了解代码,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/37365914/