CODEWITHSUNDEEP
c++stringmemory-management
Damian
Damian

Reputation: 5561

c++ Why is memory allocated when passing a string literal to a function?

I have his code:

int setAttrib(const string& name, int components){    
    // here I don't even touch 'name'
    if(components == 2) return 3;
    else return 1;
}

And I call the function this way:

setAttrib("position", 3);

I'm profiling memory with xcode profiler and in the function call std::string is making an allocation.
Why is that?

EDIT:

What's the best way to avoid that allocation? since I'm calling that function a lot, and in about 10 seconds of time I end up allocating about 10MB in that line.

Thanks.

Upvotes: 5

Views: 1247

Answers (3)

Alexander Gessler
Alexander Gessler

Reputation: 46607

Because std::string typically allocates heap memory to hold the string. In this case, a std::string is implicitly constructed from a string literal (which by itself resides in static storage). Some string implementations use a small buffer to serve small strings, but this doesn't seem to be the case here and is implementation-dependent anyway.

It doesn't matter that name is not used - basically, setAttrib("position", 3) is shorthand for setAttrib(std::string("position"), 3);, so when control enters setAttrib, the memory has already been allocated (of course, in your isolated code sample it would be possible for a compiler to inline getAttrib and then drop the string construction at all, but this is a compiler optimization, not a language feature).

Note that temporary objects created during function invocation are automatically destructed when the function returns, so there is no memory leaked.

Upvotes: 6

Christopher Creutzig
Christopher Creutzig

Reputation: 8774

You ask for a const string&, but pass in a const char*. The compiler thus needs to create a temporary object of the correct type.

The fact that "position" is not an std::string but a char const* is more of a historical accident (inherited from C, when there was no string class in C++) than a design decision, but something to keep in mind nevertheless.

Upvotes: 14

sharptooth
sharptooth

Reputation: 170479

In order to call the function the compiler needs to construct all parameters, const string& name included, the only way to do that in your case (you pass a string literal instead) is to construct a temporary std::string and this requires a heap memory allocation in most implementations. It doesn't matter if you use the value inside the function or not.

Upvotes: 5

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