C++:获取使用ofstream时打印的字符数

C++: get number of characters printed when using ofstream

本文关键字：打印 字符 ofstream 获取 C++      更新时间：2023-10-16

C fprintf()函数返回打印的字符数。使用 ofstream 写入文件时，C++中是否有类似的功能？如果可能的话，我对与 C++03 兼容的解决方案感兴趣。

例如：

ofstream file("outputFile");
file << "hello";
// Here would I like to know that five characters were printed.
file << " world";
// Here I would like to know that six characters were printed.

您要查找的是tellp()。

你可以这样使用它：

ofstream file("outputFile");
auto pos1 = file.tellp();
file << "hello";
auto pos2 = file.tellp();
std::cout << pos2 - pos1 << std::endl;

查找操作相当昂贵(主要是因为它们需要准备流才能在读取和写入之间切换(。我个人宁愿使用提供计数的过滤流缓冲区，例如：

class countbuf: public std::streambuf {
    std::streambuf* sbuf;
    std::size_t     count;
    char            buffer[256];
    int overflow(int c) {
        if (c != std::char_traits<char>::eof()) {
            *this->pptr() = c;
            this->pbump(1);
        }
        return this->sync() == -1
            ? std::char_traits<char>::eof()
            : std::char_traits<char>::not_eof(c);
    }
    int sync() {
        std::size_t size(this->pptr() - this->pbase());
        this->count += size;
        this->setp(this->buffer, this->buffer + 256);
        return size == this->sbuf->sputn(this->pbase(), this->pptr() - this->pbase())
             ? this->sbuf->pubsync(): -1;
    }
public:
    countbuf(std::streambuf* sbuf): sbuf(sbuf), count() {
        this->setp(buffer, buffer + 256);
    }
    std::size_t count() const { return count + this->pptr() - this->pbase(); }
    std::size_t reset() const {
        std::size_t rc(this->count());
        this->sync();
        this->count = 0;
        return rc;
    }
};

获得此流缓冲区后，只需安装到std::ostream中(并可能将构造打包到自定义流类中(：

countbuf     sbuf(std::cout.rdbuf()); // can't seek on this stream anyway...
std::ostream out(&sbuf);
out << "hello!n" << std::flush;
std::cout << "count=" << out.reset() << 'n';
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