仅当文件存在时才使用 fstream 读取/写入文件

Question

I'm handling a file using fstream, and I need to read and write to it. However, even using std::ios:in, the file continues to be created if it does not exist:

std::fstream file("myfile.txt", std::ios::in | std::ios::out | std::ios::app);

Any thoughts?

Thanks in advance!

Answer 1

There are different approaches, you can do it the old way

std::fstream file("myfile.txt", std::ios::in | std::ios::out); // edited after comment
if (!file)
{
    // Can't open file for some reason
}

Or you can use standard library std::filesystem::exists introduced in C++17. Read more about it .

if (std::filesystem::exists("myfile.txt")) { // Exists }

Remember that file can exist but you can fail to interact with it (read/write from/to it).

If needed:

g++ -std=c++17 yourFile.cpp -o output_executable_name

Answer 2

Read documentation carefully:

std::basic_filebuf<CharT,Traits>::open - cppreference.com

The file is opened as if by calling std::fopen with the second argument ( mode ) determined as follows:

mode	openmode & ~ate	Action if file already exists	Action if file does not exist
"r"	in	Read from start	Failure to open
"w"	out, out|trunc	Destroy contents	Create new
"a"	app, out|app	Append to file	Create new
"r+"	out|in	Read from start	Error
"w+"	out|in|trunc	Destroy contents	Create new
"a+"	out|in|app, in|app	Write to end	Create new
"rb"	binary|in	Read from start	Failure to open
"wb"	binary|out, binary|out|trunc	Destroy contents	Create new
"ab"	binary|app, binary|out|app	Write to end	Create new
"r+b"	binary|out|in	Read from start	Error
"w+b"	binary|out|in|trunc	Destroy contents	Create new
"a+b"	binary|out|in|app, binary|in|app	Write to end	Create new

This should explain everything.

Just drop app flag and you done.

https://wandbox.org/permlink/p4vLC8ane9Ndh1gN