Write C++ array to file, avoiding creating a std::string

Question

I have a struct representing ASCII data:

struct LineData
{
    char _asciiData[256];
    uint8_t _asciiDataLength;
}

created using:

std::string s = "some data here";
memcpy(obj._asciiData, s.length());
obj._asciiDataLength = s.length();

How do I write the char array to file as ASCII, in the lowest latency? I want to avoid the intermediary stage of creating a temporary std::string.

I tried:

file.write((char *)obj._asciiData, sizeof(obj._asciiDataLength));
file << std::endl;

but my file just contains '0' each line.

@Mgetz I'm not writing binary, I'm writing ASCII but without strings? — intrigued_66
– intrigued_66, Commented Jun 27, 2019 at 13:12
then insofar as the char array is zero terminated you can just use operator << to output it doesn't need a string? — Mgetz
– Mgetz, Commented Jun 27, 2019 at 13:14
You write memcpy(obj._asciiData, s.length()); shouldn't be something like memcpy(obj._asciiData, s.c_str(), s.length()); ? You might want to consider using strncpy cplusplus.com/reference/cstring/strncpy — CuriouslyRecurringThoughts
– CuriouslyRecurringThoughts, Commented Jun 27, 2019 at 13:20

Hatted Rooster · Accepted Answer · 2019-06-27 13:11:17Z

4

That's because sizeof(obj._asciiDataLength) is probably 1 on your system so only one character is written. You want the actual length, not the size of the uint8_t:

file.write(obj._asciiData, obj._asciiDataLength);

edited Jun 27, 2019 at 13:11

answered Jun 27, 2019 at 13:10

Hatted Rooster

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7 Comments

eerorika Over a year ago

Also, the explicit cast is redundant.

Hatted Rooster Over a year ago

@eerorika Right, I only focused on the faulty part but it is indeed redundant.

Mgetz Over a year ago

Technically insofar as _asciiData is zero terminated they should be able to just use operator << too

Bob__ Over a year ago

@Mgetz Note how the OP creates that struct: memcpy(obj._asciiData, s.length());. Typo (the source is missing, I'd assume s.c_str()) apart, it uses length(), not length() + 1.

Mgetz Over a year ago

@Bob__ I suspect we're seeing an odd snapshot where there are better ways of doing things. Hence the "insofar". I really don't think we have a clear picture.

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