c++ - 文件输入和指针所有权语义

Question

我知道有两种方法可以将文件内容读取为 C 风格的字符串。我需要 C 风格的字符串而不是 unique_ptr<string> 的原因是因为我会将此数据传递给 C 函数， gumbo_parse() 其参数是 C 风格的字符串，这样我就避免了 unique_ptr<string> 的开销.我愿意接受反对我的决定的争论。

std::ifstream ifs(bf::path("..."));                                              // Input file stream

第一种方式:

std::filebuf * ptr_fbuff = ifs.rdbuf();                                         // Buffer to file contents.
std::size_t fbuff_size = ptr_fbuff->pubseekoff(0, ifs.end, std::ios_base::in);  // Get offset from 0 to end of buffer.
ptr_fbuff->pubseekpos(0, std::ios_base::in);                                    // Move buffer position back to beginning.
char * buffer = new char[fbuff_size];                                           // Buffer to store file data.
ptr_fbuff->sgetn(buffer, fbuff_size);                                           // Get file data.

第二种方式:

std::stringstream sstm_buffer;                                                  // Buffer to file contents.
sstm_buffer << ifs.rdbuf();                                                     // Read file contents to sstm.
const std::string & str_buffer = sstm_buffer.str();                             // Get underlying string from sstm.
char * buffer = new char[str_buffer.size()];                                    // Buffer to store file data.
str_buffer.copy(buffer, str_buffer.size());                                     // Copy file contents to buffer.

做事:

GumboOutput * ptr_outpt = gumbo_parse(buffer);
//...

关闭文件:

gumbo_destroy_output(&kGumboDefaultOptions, ptr_outpt);
ifs.close();                                                                    // Close stream to file.
delete[] buffer;                                                                // Delete allocated buffer.

两者在内存成本和速度方面有何不同。很明显，一个在将内容放入 C 风格字符串之前使用 Stringstream 作为缓冲区，而另一个在将内容放入 C 风格字符串之前使用 filebuf。

现在回答我的第二个问题。所有权语义和内存分配。 ifstream 是否在堆中为从 rdbuf 返回的缓冲区分配任何内存？我是否负责删除从 rdbuf 返回的缓冲区？如果没有，假设我正在读取一个巨大的文件...这不是有可能在堆栈中存储大量数据吗？

编辑:

std::unique_ptr<std::string> mytype::my_func(const std::string & path)
{
    std::ifstream ifs(path);                                            // Input file stream
    std::stringstream sstm_buffer;                                      // Buffer to file contents.
    sstm_buffer << ifs.rdbuf();                                         // Read file contents to sstm.
    ifs.close();                                                        // Close stream to file.
    auto ptr_buffer = std::make_unique<std::string>(sstm_buffer.str()); // Pointer to copy of sstm buffer contents.
    return std::move(ptr_buffer);                                       // Return pointer.
}

编辑2:

std::string mytype::my_func(const std::string & path) const
{
    std::ifstream ifs(path);
    std::stringstream sstm_buf;
    sstm_buf << ifs.rdbuf();
    ifs.close();
    return sstm_buf.str();
}

编辑3:

std::string psclient::file_get(const std::string & path) const
{
    std::ifstream ifs(path);    // Stream to file (Automatically close() on ~).
    std::ostringstream reader;  // Stream to read file contents.
    reader << ifs.rdbuf();      // Read in file contents.
    return reader.str();        // Return a move instead of copy (Done implicitly).
}

Answer 1

The reason I need a C-style string and not a unique_ptr is because I will be passing this data to a C function

你仍然可以获得一个C风格的指针，例如

#include <iostream>
#include <string>
#include <memory>

void print_C_style_string(const char* psz)
{
    printf("str = %s", psz);
}

int main()
{
    std::unique_ptr<std::string> p{new std::string{"This is a string!"}};
    print_C_style_string(p.get()->c_str());
    return 0;
}

unique_ptr 没有开销。摘自 Bjarne Stroustrup 的“The C++ Programming Language”:

unique_ptr is a very lightweight mechanism with no space or time overhead compared to correct use of a built-in pointer.

将文件逐行读入字符串的简单方法:

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main()
{
    ifstream fin("my_file.txt", ios::in);
    if(!fin){
        printf("Error opening file.");
        return 1;
    }

    while(fin.peek() != EOF){
        string s;
        getline(fin, s);
        cout << s << std::endl;
    }

    fin.close();

    return 0;
}