How to read a string from a binary file?

Question

I am trying to read a string(ver) from a binary file. the number of characters(numc) in the string is also read from the file.This is how I read the file:

 uint32_t numc;
 inFile.read((char*)&numc, sizeof(numc));
 char* ver = new char[numc];
 inFile.read(ver, numc);
 cout << "the version is: " << ver << endl;

what I get is the string that I expect plus some other symbols. How can I solve this problem?

it is not null terminated in the file. You will need to store it in a length-aware string storage, like for example std::string, or provide the null character yourself. See also stackoverflow.com/questions/27061489/… — Kenny Ostrom
– Kenny Ostrom, Commented Jul 9, 2020 at 19:18

john · Accepted Answer · 2020-07-09 19:23:37Z

1

A char* string is a nul terminated sequence of characters. Your code ignores the nul termination part. Here's how it should look

uint32_t numc;
inFile.read((char*)&numc, sizeof(numc));
char* ver = new char[numc + 1]; // allocate one extra character for the nul terminator
inFile.read(ver, numc);
ver[numc] = '\0'; // add the nul terminator
cout << "the version is: " << ver << endl;

Also sizeof(numc) not size(numc) although maybe that's a typo.

edited Jul 9, 2020 at 19:23

answered Jul 9, 2020 at 19:18

john

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6 Comments

user4581301 Over a year ago

If this doesn't work for you, make sure the endian of numc matches the endian of the file

SJ93 Over a year ago

You were right. I'd forgotten the null character. the problem is solved. Thanks.

Remy Lebeau Over a year ago

Don't forget to delete[] ver when you are done using it. Better to use std::string instead, or at least std::unique_ptr<char[]>

SJ93 Over a year ago

@RemyLebeau I don't understand why is that a problem?

Remy Lebeau Over a year ago

@MahshadJavidan Memory leak vs no memory leak. You have to delete whatever you allocate with new. std::unique_ptr will handle that for you. And you don't have to worry about that at all with std::string.

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