Does std::ofstream truncate or append by default?

Question

If you call the std::ofstream constructor without openmode flags, the default flag is ios_base::out. But does this imply ios_base::trunc or ios_base::app?

In other words, if you already have a non-empty file "past.txt" in your file system and you call

std::ofstream stream( "past.txt" );
stream << "new content";

will "new content" be appended to the prior contents of "past.txt" or will it replace the prior contents?

Answer 1

The short version

It truncates by default.

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The medium version

The standard is basically spaghetti on this, but it eventually boils down to saying that it's the equivalent of saying fopen(const char*, "w") (27.9.1.4 [filebuf.members]), which then points us towards the ISO C 7.9 standard.

Checking that out provides us with §7.19.5.3, "The fopen function", which specifies the behavior when "w" is passed:

w truncate to zero length or create text file for writing

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The long version

If you'd like to follow the spaghetti trail yourself, start with 27.9.1.11 [ofstream.cons], which describes the constructor's behavior as

Effects: Constructs an object of class basic_ofstream<charT,traits>, initializing the base class with basic_ostream(&sb) and initializing sb with basic_filebuf<charT,traits>()) (27.7.3.2, 27.9.1.2), then calls rdbuf()->open(s, mode|ios_base::out). If that function returns a null pointer, calls setstate(failbit).

Where rdbuf() returns basic_filebuf<charT,traits>* (27.9.1.13 [ofstream])

Which leads us to 27.9.1.1 [filebuf], or more specifically, 27.9.1.4 [filebuf.members] , which describes the open function:

basic_filebuf<charT,traits>* open(const char* s, ios_base::openmode mode);

as

Effects: If is_open() != false, returns a null pointer. Otherwise, initializes the filebuf as required. It then opens a file, if possible, whose name is the NTBS s (as if by calling std::fopen(s,modstr)). The NTBS modstr is determined from mode & ~ios_base::ate as indicated in Table 132. If mode is not some combination of flags shown in the table then the open fails.

NTBS: Null-terminated byte-string

Table 132 describes equivalence rules between C++ ios_base::openmode and C-style stdio strings:

Table 132 — File open modes
|
|   'ios_base' flag combination   | 'stdio' equivalent |
| binary | in | out | trunc | app |                    |
|        |    |  +  |       |     |        "w"         |
|                     etc...                           |

Which leads us to a footnote on the same page that states:

...the function signatures fopen(const char*, const char*) and fseek(FILE*, long, int) are declared, in <cstdio> (27.9.2).

Which sends us, predictably, to 27.9.2 [c.files], which provides the nearly useless Table 134, but then references the C standard:

See also: ISO C 7.9, Amendment 1 4.6.2.

Which I talk about in the main portion of this answer.