Question

我试图在我正在编写特定数据的文件中发现不需要的尾随结束数据的原因，并且不相信我在写入文件时出错。

输出如下：

building     room_numbr   capacity

packard    | 101        | 500        |
painter    | 514        | 10         |
ÿÿÿÿÿÿÿÿÿÿ | Attempt to seek file pointer error

Attempt to seek file pointer error是正常的，因为它表示在尝试在无效流上移动文件指针时抛出的异常。但是，使用10或20个字节来写入数据时，ÿÿÿÿÿÿÿÿÿÿ在固定大小的文件格式中不正常也不是预期的。

在此处创建文件：

BinarySearchFile::BinarySearchFile(std::string file_name){

    // concatenate extension to fileName
    file_name += ".dat";

    // form complete table data filename
    data_file_name = file_name;

    // create or reopen table data file for reading and writing
    binary_search_file.open(data_file_name,  std::ios::out | std::ios::in | std::ios::app); 

    if(!binary_search_file.is_open()){

        binary_search_file.clear();
        binary_search_file.open(data_file_name, std::ios::out);
        binary_search_file.close();
        binary_search_file.open(data_file_name, std::ios::out | std::ios::in | std::ios::app);
    }

    try{
        if(binary_search_file.fail()){
            throw CustomException("Unspecified table data file error");
        }
    }
    catch (CustomException &custom_exception){  // Using custom exception class
        std::cout << custom_exception.what() << std::endl;
        return;
    }   

}

将数据写入文件

void BinarySearchFile::writeT(std::string attribute){
try{
    if(binary_search_file){
        for(auto start = attribute.begin(); start != attribute.end();  ++start){
            binary_search_file.put(' ');
            binary_search_file.put(*start);
        }
        binary_search_file.flush();
        /*
        attribute.resize(attribute.length() * 2);
        const char *write_this = attribute.data();
        binary_search_file.write(write_this, attribute.length());
        */
    }else if(binary_search_file.fail()){
        throw CustomException("Attempt to write attribute error");
    }

}
catch(CustomException &custom_exception){  // Using custom exception class
    std::cout << custom_exception.what() << std::endl;
    return;
}
}

在此处阅读数据文件：

   std::string BinarySearchFile::readT(long file_pointer_location, long size_of_data) 
{
try{
    if(binary_search_file){

        std::string data = "";
        binary_search_file.seekp(file_pointer_location);
        binary_search_file.seekg(file_pointer_location);
        while (size_of_data > 0 ){
            binary_search_file.get();
            data += binary_search_file.get();
            size_of_data -= 2;
        }

        /*
        char data[20];
        binary_search_file.seekp(filePointerLocation);
        binary_search_file.seekg(filePointerLocation);
        binary_search_file.read(data, sizeOfData);
        */
        return data;
    }else if(binary_search_file.fail()){
        throw CustomException("Attempt to read attribute error");
    }

}
catch(CustomException &custom_exception){  // Using custom exception class
    std::cout << custom_exception.what() << std::endl;
}

}

读取文件并将结果打印到屏幕的代码：

while(true){

    //reinitialize the catalog pointer to the beginning 
    catalog->setPointerBegin();

    //display data
    do{
        if (boost::iequals((domain = catalog->getAttributeDomain()), "string")){
            if(dataFile->binary_search_file_status()){
                std::cout << dataFile->read_data(filePointer, 20) << " | ";
                if (!writer_.fail())
                    writer_ << dataFile->read_data(filePointer, 20) << " | ";
            }
            else{
                std::cout << "\n";
                if (!writer_.fail())
                    writer_ << "\n";

                        return true;
                }
                // update the file pointer
                filePointer += 20;
                dataFile->set_file_pointer(filePointer);
            }
            else{
                if(dataFile->binary_search_file_status()){
                    std::cout << dataFile->read_data(filePointer, 10);
                    if (!writer_.fail())
                        writer_ << dataFile->read_data(filePointer, 10);
                    for(int i = 0; i < 5; i++){
                            std::cout << " ";
                            if (!writer_.fail())
                                writer_ << " ";
                        }
                        std::cout << " | ";
                        if (!writer_.fail()){
                            writer_ << " | ";
                    }
                }
                else{
                    std::cout << "\n";
                    if (!writer_.fail()){
                        writer_ << "\n";
                    }
                    return true;
                }
                // update the file pointer
                filePointer += 10;

                }       


            } while(catalog->traverseForward() != nullptr);

            std::cout << "\n";
            if (!writer_.fail())
                writer_ << "\n";

    }

}

Answer 1

std::ifstream::get在失败时返回std::char_traits<char>::eof，其通常具有int值-1。如果您盲目地将其解释为有效字符并转换为char，您将获得'\xff'，其中ISO-8859-15为ÿ。

当您从文件中读取字符时，尤其是在搜索后，您应该检查eof和/或eofbit。

期待eof，而不是eof附近的任意数据

1 个答案: