HOW TO：使用訊息區篩選條件

本文件示範如何使用篩選函式，讓非同步訊息區塊根據訊息裝載來接受或拒絕訊息。

當您建立訊息區塊物件如 concurrency::unbounded_buffer、 concurrency::call，或 concurrency::transformer，您可以提供 filter 函式 ，會決定訊息區接受或拒絕一則訊息。篩選函式是確保訊息區塊只接收特定值的實用方法。

篩選函式很重要，因為它們可讓您連接訊息區塊以形成「資料流程網路」(Dataflow Network)。在資料流程網路中，訊息區塊只會處理符合特定準則的訊息，藉以控制資料流程。相較之下，在控制流程模型中，資料流程是透過條件陳述式、迴圈等控制結構來規範。

本文件提供如何使用訊息篩選條件的基本範例。如需使用訊息篩選條件和資料流程模型以連接訊息區塊的其他範例，請參閱逐步解說：建立資料流程代理程式和逐步解說：建立影像處理網路。

範例

請考慮下列函式 count_primes，這說明不篩選傳入訊息的訊息區塊基本用法。此訊息區塊會將質數附加至 std::vector 物件。count_primes 函式會傳送數個數字至訊息區塊，從訊息區塊接收輸出值，然後將質數列印至主控台。

// 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 物件會處理所有輸入值，不過，它只需要質數值。雖然應用程式撰寫方式可讓訊息傳送者只傳送質數，但訊息接收者的需求不一定是已知。

下列函式 count_primes_filter 會執行與 count_primes 函式相同的工作。不過，這個版本中 transformer 物件使用篩選函式，只接受質數值。執行動作的函式只接收質數，因此，它不必呼叫 is_prime 函式。

因為 transformer 物件只接收質數，所以 transformer 物件本身可以保存質數。換言之，這個範例中的 transformer 物件不需要將質數加入至 vector 物件。

// 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 物件現在只會處理質數值。在前述範例中，transformer 物件會處理所有訊息。因此，前述範例必須接收與所傳送相同數目的訊息。本範例使用的結果 concurrency::send 函式來判斷可接受來自多少訊息transformer物件。如果訊息緩衝區接受訊息，send 函式會傳回 true，如果訊息緩衝區拒絕訊息則傳回 false。因此，訊息緩衝區接受訊息的次數會符合質數計數。

下列程式碼顯示完整範例。範例會呼叫 count_primes 函式和 count_primes_filter 函式。

// 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:
    9973
    9349
    9241
    8893
    1297
    7127
    8647
    3229
    With filtering:
    The following numbers are prime:
    9973
    9349
    9241
    8893
    1297
    7127
    8647
    3229
    */
}

這個範例會產生下列輸出：

Without filtering:
The following numbers are prime:
9973
9349
9241
8893
1297
7127
8647
3229
With filtering:
The following numbers are prime:
9973
9349
9241
8893
1297
7127
8647
3229

編譯程式碼

將範例程式碼複製並貼上它在 Visual Studio 專案中，或將它貼在檔名為堅壁 filter.cpp ，然後執行下列命令，Visual Studio 的命令提示字元] 視窗中。

cl.exe /EHsc primes-filter.cpp

穩固程式設計

篩選函式可以是 Lambda 函式、函式指標或函式物件。每個篩選函式都採用下列其中一個形式：

bool (_Type)
bool (_Type const &)

若要排除不必要的資料複製，當您的彙總型別是以傳值方式傳輸時，請使用第二個形式。

請參閱

工作

逐步解說：建立資料流程代理程式

逐步解說：建立影像處理網路

參考

transformer 類別

概念

非同步代理程式程式庫