CS253 Functions

Functions

By & large, C++ functions look like Java functions:

int sum(int a, int b) {
    return a+b;
}

int main() {
    cout << sum(2, 40) << '\n';
}

Default Parameters

Functions can have default trailing arguments:

int sum(int a, int b = 1) {
    return a+b;
}

int main() {
    cout << sum(2, 40) << '\n'
         << sum(8) << '\n';
}

42
9

Default Parameters

All default arguments must be piled up at the end:

int sum(int a = 123, int b = 456) {
    return a+b;
}

int main() {
    cout << sum(2, 40) << '\n'
         << sum(8) << '\n'
         << sum() << '\n';
}

42
464
579

Function Overloading

Like Java, C++ can have multiple versions of the same function that take different arguments:

int sum(int a, int b) { return a+b; } 
int sum(int a) { return a+456; } 
int sum() { return 123+456; } 

int main() {
    cout << sum(2, 40) << '\n'
         << sum(8) << '\n'
         << sum() << '\n';
}

42
464
579

Function Overloading

The types don’t have to be the same, either:

int sum(int a, int b) { return a+b; } 
double sum(int a) { return a+4.5; } 
string sum(double) { return "I can’t deal with double"; } 
const char *sum() { return "You are a bonehead"; } 

int main() {
    cout << sum(2, 40) << '\n'
         << sum(8) << '\n'
         << sum(123.456) << '\n'
         << sum() << '\n';
}

42
12.5
I can’t deal with double
You are a bonehead

Implementation

So, how is this accomplished?
How can there actually be four versions of the function sum in the executable file?
sum is sum, right?
No.

Name Mangling

It’s called name mangling.
Inside the executable file, the function isn’t really called sum.
Instead, it’s called sum-that-takes-an-int-argument.
Perhaps it’s more compact than that.

Return type doesn’t matter

The function's return type is not part of the mangled name.
Therefore, a function can’t be overloaded on return type alone.

int foo() { return 1; }
double foo() { return 2.0; }
int main() {
    return 0;
}

c.cc:2: error: ambiguating new declaration of 'double foo()'

Name Mangling

$ cat ~cs253/Examples/overload.cc
int sum(int a, int b) { return a+b; }
double sum(int a) { return a+4.5; }
const char *sum() { return "You are a bonehead"; }
bool sum(double, int, short) { return false; }

$ g++ -c ~cs253/Examples/overload.cc

$ nm --numeric-sort overload.o
0000000000000000 T _Z3sumii
0000000000000014 T _Z3sumi
000000000000002e T _Z3sumv
0000000000000039 T _Z3sumdis

$ nm --numeric-sort --demangle overload.o
0000000000000000 T sum(int, int)
0000000000000014 T sum(int)
000000000000002e T sum()
0000000000000039 T sum(double, int, short)

CS253: Software Development with C++

Fall 2018

Functions

CS253 Functions

Functions

Default Parameters

Default Parameters

Function Overloading

Function Overloading

Implementation

Name Mangling

Return type doesn’t matter

Name Mangling