在C++中，可以使用以下方法來統計單詞個數：

1. 使用字符串流（stringstream）和循環來逐個讀取單詞并計數。

#include <iostream>
#include <sstream>

int countWords(const std::string& sentence) {
    std::stringstream ss(sentence);
    std::string word;
    int count = 0;

    while (ss >> word) {
        count++;
    }

    return count;
}

int main() {
    std::string sentence = "Hello world, this is a sentence.";
    int wordCount = countWords(sentence);
    std::cout << "Word count: " << wordCount << std::endl;

    return 0;
}

2. 使用字符串的find_first_of函數和循環來逐個找到單詞并計數。

#include <iostream>
#include <string>

int countWords(const std::string& sentence) {
    int count = 0;
    size_t pos = sentence.find_first_of(" \t\n");

    while (pos != std::string::npos) {
        count++;
        pos = sentence.find_first_of(" \t\n", pos + 1);
    }

    return count + 1;
}

int main() {
    std::string sentence = "Hello world, this is a sentence.";
    int wordCount = countWords(sentence);
    std::cout << "Word count: " << wordCount << std::endl;

    return 0;
}

這些方法可以根據空格、制表符、換行符等作為單詞的分隔符進行統計。請根據實際需求選擇使用。