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本文档演示了如何使用筛选器函数，使异步消息块能够根据消息的有效负载接受或拒绝消息。
创建消息块对象(例如 concurrency::unbounded_buffer、concurrency::call 或 concurrency::transformer)时，可以提供筛选器函数，用于确定消息块是接受还是拒绝消息。 筛选器函数是保证消息块仅接收特定值的有效方式。
筛选器函数很重要，因为它们使你能够连接消息块以形成数据流网络。 在数据流网络中，消息块通过仅处理满足特定标准的消息来控制数据流。 将数据流网络与控制流模型进行比较，在后者中，数据流是通过使用条件语句、循环等控制结构来调节的。
本文档提供了有关如何使用消息筛选器的基本示例。
示例：count_primes 函数
考虑以下函数 count_primes，该函数说明了不筛选传入消息的消息块的基本用法。 此消息块将质数追加到 std::vector 对象。 count_primes 函数将几个数字发送到消息块，从消息块接收输出值，并将这些数字打印到控制台。
// Illustrates usage of a message buffer that does not use filtering.
void count_primes(unsigned long random_seed)
{// Holds prime numbers.vector<unsigned long> primes;// Adds numbers that are prime to the vector object.transformer<unsigned long, unsigned long> t([&primes](unsigned long n) -> unsigned long{if (is_prime(n)){primes.push_back(n);}return n;});// Send random values to the message buffer.mt19937 generator(random_seed);for (int i = 0; i < 20; ++i){send(t, static_cast<unsigned long>(generator()%10000));}// Receive from the message buffer the same number of times// to ensure that the message buffer has processed each message.for (int i = 0; i < 20; ++i){receive(t);}// Print the prime numbers to the console.wcout << L"The following numbers are prime: " << endl;for(unsigned long prime : primes){wcout << prime << endl;}
}transformer 对象处理所有输入值；但是，它只需要那些为质数的值。 尽管可以编写应用程序以使消息发送方仅发送质数，但并不能始终得知消息接收方的要求。
示例：count_primes_filter 函数
以下函数 count_primes_filter 执行与 count_primes 函数相同的任务。 但是，此版本中的 transformer 对象使用筛选器函数以便仅接受那些为质数的值。 执行该操作的函数仅接收质数；因此，它不必调用 is_prime 函数。
由于 transformer 对象仅接收质数，所以 transformer 对象本身可以保存质数。 换言之，此示例中的 transformer 对象不需要将质数添加到 vector 对象。
// Illustrates usage of a message buffer that uses filtering.
void count_primes_filter(unsigned long random_seed)
{// Accepts numbers that are prime.transformer<unsigned long, unsigned long> t([](unsigned long n) -> unsigned long{// The filter function guarantees that the input value is prime.// Return the input value.return n;},nullptr,[](unsigned long n) -> bool{// Filter only values that are prime.return is_prime(n);});// Send random values to the message buffer.mt19937 generator(random_seed);size_t prime_count = 0;for (int i = 0; i < 20; ++i){if (send(t, static_cast<unsigned long>(generator()%10000))){++prime_count;}}// Print the prime numbers to the console. wcout << L"The following numbers are prime: " << endl;while (prime_count-- > 0){wcout << receive(t) << endl;}
}transformer 对象现在仅处理那些为质数的值。 在前面的示例中，transformer 对象处理所有消息。 因此，前面的示例必须接收与其发送的相同数量的消息。 此示例使用 concurrency::send 函数的结果来确定要从 transformer 对象接收的消息数。 send 函数在消息缓冲区接受消息时返回 true，在消息缓冲区拒绝消息时返回 false。 因此，消息缓冲区接受消息的次数与质数的计数相匹配。
示例：已完成的消息块筛选器代码示例
以下代码显示完整示例。 该示例同时调用 count_primes 函数和 count_primes_filter 函数。
// primes-filter.cpp
// compile with: /EHsc
#include <agents.h>
#include <algorithm>
#include <iostream>
#include <random>using namespace concurrency;
using namespace std;// Determines whether the input value is prime.
bool is_prime(unsigned long n)
{if (n < 2)return false;for (unsigned long i = 2; i < n; ++i){if ((n % i) == 0)return false;}return true;
}// Illustrates usage of a message buffer that does not use filtering.
void count_primes(unsigned long random_seed)
{// Holds prime numbers.vector<unsigned long> primes;// Adds numbers that are prime to the vector object.transformer<unsigned long, unsigned long> t([&primes](unsigned long n) -> unsigned long{if (is_prime(n)){primes.push_back(n);}return n;});// Send random values to the message buffer.mt19937 generator(random_seed);for (int i = 0; i < 20; ++i){send(t, static_cast<unsigned long>(generator()%10000));}// Receive from the message buffer the same number of times// to ensure that the message buffer has processed each message.for (int i = 0; i < 20; ++i){receive(t);}// Print the prime numbers to the console.wcout << L"The following numbers are prime: " << endl;for(unsigned long prime : primes){wcout << prime << endl;}
}// Illustrates usage of a message buffer that uses filtering.
void count_primes_filter(unsigned long random_seed)
{// Accepts numbers that are prime.transformer<unsigned long, unsigned long> t([](unsigned long n) -> unsigned long{// The filter function guarantees that the input value is prime.// Return the input value.return n;},nullptr,[](unsigned long n) -> bool{// Filter only values that are prime.return is_prime(n);});// Send random values to the message buffer.mt19937 generator(random_seed);size_t prime_count = 0;for (int i = 0; i < 20; ++i){if (send(t, static_cast<unsigned long>(generator()%10000))){++prime_count;}}// Print the prime numbers to the console. wcout << L"The following numbers are prime: " << endl;while (prime_count-- > 0){wcout << receive(t) << endl;}
}int wmain()
{const unsigned long random_seed = 99714;wcout << L"Without filtering:" << endl;count_primes(random_seed);wcout << L"With filtering:" << endl;count_primes_filter(random_seed);/* Output:99739349924188931297712786473229With filtering:The following numbers are prime:99739349924188931297712786473229*/
}可靠编程
筛选器函数可以是 lambda 函数、函数指针或函数对象。 每个筛选器函数采用以下格式之一：
bool (T)
bool (T const &)为了消除不必要的数据复制，需要按值传输聚合类型时，请使用第二种格式。