C# allows to mark function argument as output only:
void func(out int i)
{
i = 44;
}
Is it possible to do something similar in C/C++? This could improve optimization. Additionally is should silence out warnings "error: 'myVar' may be used uninitialized in this function", when variable is not initialized and then passed to function as output argument.
I use gcc/g++ (currently 4.4.7) to compile my code.
Edit: I know about pointers and references, this is not what I am looking for. I need something like this:
void func(int* __attribute__((out)) i)
{
*i = 44;
}
void func2()
{
int myVal; // gcc will print warning: 'myVar' may be used uninitialized in this function
func(&myVal);
//...
}
Edit 2: Some extra code is needed to reproduce warning "'myVar' may be used uninitialized in this function". Additionally you have to pass -Wall -O1 to gcc.
void __attribute__((const)) func(int* i)
{
*i = 44;
}
int func2()
{
int myVal; // warning here
func(&myVal);
return myVal;
}
"Is it possible to do something similar in C/C++?"
Not really. Standard c++ or c doesn't support such thing like a output only parameter.
In c++ you can use a reference parameter to get in/out semantics:
void func(int& i) {
// ^
i = 44;
}
For c you need a pointer, to do the same:
void func(int* i) {
// ^
*i = 44;
// ^
}
Note that there are no distinctions between out and in&out parameters, unless you use a const reference (which means input only):
void func(const int& i) {
// ^^^^^
i = 44;
}
When you want to pass an arg as output in C++ you pass it as a reference :
void func(int &i)
{
i = 44;
}
and you must initialize it before doing anything on it.
Note :
You can specify when you want the arg to be just an input with a const ref :
void func(const int &i)
{
i = 44;
}
Related
Consider having these four functions in one C++ program:
void a(int val)
{
cout<<val;
}
void a(int &val)
{
cout<<val;
}
void a(int *val)
{
cout<<val;
}
void a(double val)
{
cout<<val;
}
Few Questions i have are:
Is there going to be any error in the code? or are they all overloaded without any error.
Can you tell me how to call all of these four functions correctly? My try was below:
int iTmp;
int *pTmp;
double dTmp;
a(iTmp);
a(iTmp);
a(pTmp);
a(dTmp);
The only problem are the functions:
void a(int &val)
and
void a(int val)
The compiler will create the following errors:
Compilation error time: 0 memory: 3140 signal:0
prog.cpp: In function 'int main()':
prog.cpp:28:8: error: call of overloaded 'a(int&)' is ambiguous
a(iTmp);
^
Because he cant distinguish both, if you remove one of them the compilation succeeds
See Example:
#include <iostream>
using namespace std;
void a(int val)
{
cout<<val;
}
void a(int *val)
{
cout<<val;
}
void a(double val)
{
cout<<val;
}
int main() {
int iTmp = 0;
int *pTmp = 0;
double dTmp = 0.0;
a(iTmp);
a(iTmp);
a(pTmp);
a(dTmp);
return 0;
}
See working example:
http://ideone.com/WRZUoW
Your code will compile but calling
int iTmp;
a(iTmp);
will yield an ambiguous overload resolution call since both
void a(int val)
{
cout<<val;
}
void a(int &val)
{
cout<<val;
}
will match.
The reason for this lies in the standard:
[over.match.best]
A viable function F1 is defined to be a better function than another viable function F2 if for all arguments i, ICSi(F1) is not a worse conversion sequence than ICSi(F2)
int->int
int->int&
these are standard conversion sequences and the entire list of [over.ics.rank]p3.2 is checked without success for a better conversion sequence (exact match in both cases)
They are thus deemed "undistinguishable".
A word of advice: the variable are also not initialized and (if automatic variables) even if your code compiled, the output would be undefined.
The compiler must be able to know what function to call.
With
int i = 12;
a(i);
both a(int i) and c(int& i) are acceptable candidates and compiler throws an error.
and
void a(const int& i) { ... }
will suffer from same problem.
These two function will cause issues
void a(int val)
{
cout<<val;
}
void a(int &val)
{
cout<<val;
}
Your call a(iTmp); have two possible candidates. Compiler should shown an error for these. Otherwise pointer argument(int *val)and double argument(double val) are okay.
The most obvious problem is that
void a(int val) {
and
void a(int& val) {
are ambiguous when called with iTmp.
Aside from that, depending on whether the variables are global or local ones, the code is using uninitialized variables -> UB and whether or not,
void a(int* val) {
std::cout << val << std::endl;
}
is overloaded correctly is up for debate. I'd assume that it should be std::cout << *val << std::endl;, but that of course depends on what the function is supposed to do (although a function that prints an address would usually be parameterized by (void* or char*);
For example, there is a class foo :
class foo {
public:
foo (int = 10);
.....
}
The prototype of the constructor has "int = 10" inside. So, what does it mean? Int is just an integer type, isn't? So, isn't that illegal to assign a value to it? I was trying to find such an example in Prata's book, and everywhere else, but I didn't find the explanation.
You can omit the name of parameter in function declaration (in definition too), but still you are able to specify default value of that parameter.
Consider:
void f(int x = 10) {
printf("%d\n", x);
}
void g(int = 10);
void g(int x) {
printf("%d\n", x);
}
int main() {
f();
g();
return 0;
}
Result:
10
10
The same situation is in the constructor case.
So, isn't that illegal to assign a value to it?
There is absolutely no assignment involved here. The = character can have quite different meanings in C++:
Assignment: i = 0;
Initialisation: int i = 0;
Making a member function pure virtual: virtual void f() = 0;
Specifiying default arguments: void f(int i = 0);
The latter is what you have encountered. A constructor can have default arguments like any other normal function.
When should I explicitly write this->member in a method of
a class?
Usually, you do not have to, this-> is implied.
Sometimes, there is a name ambiguity, where it can be used to disambiguate class members and local variables. However, here is a completely different case where this-> is explicitly required.
Consider the following code:
template<class T>
struct A {
T i;
};
template<class T>
struct B : A<T> {
T foo() {
return this->i; //standard accepted by all compilers
//return i; //clang and gcc will fail
//clang 13.1.6: use of undeclared identifier 'i'
//gcc 11.3.0: 'i' was not declared in this scope
//Microsoft C++ Compiler 2019 will accept it
}
};
int main() {
B<int> b;
b.foo();
}
If you omit this->, some compilers do not know how to treat i. In order to tell it that i is indeed a member of A<T>, for any T, the this-> prefix is required.
Note: it is possible to still omit this-> prefix by using:
template<class T>
struct B : A<T> {
int foo() {
return A<T>::i; // explicitly refer to a variable in the base class
//where 'i' is now known to exist
}
};
If you declare a local variable in a method with the same name as an existing member, you will have to use this->var to access the class member instead of the local variable.
#include <iostream>
using namespace std;
class A
{
public:
int a;
void f() {
a = 4;
int a = 5;
cout << a << endl;
cout << this->a << endl;
}
};
int main()
{
A a;
a.f();
}
prints:
5
4
There are several reasons why you might need to use this pointer explicitly.
When you want to pass a reference to your object to some function.
When there is a locally declared object with the same name as the member object.
When you're trying to access members of dependent base classes.
Some people prefer the notation to visually disambiguate member accesses in their code.
Although I usually don't particular like it, I've seen others use this-> simply to get help from intellisense!
There are few cases where using this must be used, and there are others where using the this pointer is one way to solve a problem.
1) Alternatives Available: To resolve ambiguity between local variables and class members, as illustrated by #ASk.
2) No Alternative: To return a pointer or reference to this from a member function. This is frequently done (and should be done) when overloading operator+, operator-, operator=, etc:
class Foo
{
Foo& operator=(const Foo& rhs)
{
return * this;
}
};
Doing this permits an idiom known as "method chaining", where you perform several operations on an object in one line of code. Such as:
Student st;
st.SetAge (21).SetGender (male).SetClass ("C++ 101");
Some consider this consise, others consider it an abomination. Count me in the latter group.
3) No Alternative: To resolve names in dependant types. This comes up when using templates, as in this example:
#include <iostream>
template <typename Val>
class ValHolder
{
private:
Val mVal;
public:
ValHolder (const Val& val)
:
mVal (val)
{
}
Val& GetVal() { return mVal; }
};
template <typename Val>
class ValProcessor
:
public ValHolder <Val>
{
public:
ValProcessor (const Val& val)
:
ValHolder <Val> (val)
{
}
Val ComputeValue()
{
// int ret = 2 * GetVal(); // ERROR: No member 'GetVal'
int ret = 4 * this->GetVal(); // OK -- this tells compiler to examine dependant type (ValHolder)
return ret;
}
};
int main()
{
ValProcessor <int> proc (42);
const int val = proc.ComputeValue();
std::cout << val << "\n";
}
4) Alternatives Available: As a part of coding style, to document which variables are member variables as opposed to local variables. I prefer a different naming scheme where member varibales can never have the same name as locals. Currently I'm using mName for members and name for locals.
Where a member variable would be hidden by
a local variable
If you just want
to make it explictly clear that you
are calling an instance method/variable
Some coding standards use approach (2) as they claim it makes the code easier to read.
Example:
Assume MyClass has a member variable called 'count'
void MyClass::DoSomeStuff(void)
{
int count = 0;
.....
count++;
this->count = count;
}
One other case is when invoking operators. E.g. instead of
bool Type::operator!=(const Type& rhs)
{
return !operator==(rhs);
}
you can say
bool Type::operator!=(const Type& rhs)
{
return !(*this == rhs);
}
Which might be more readable. Another example is the copy-and-swap:
Type& Type::operator=(const Type& rhs)
{
Type temp(rhs);
temp.swap(*this);
}
I don't know why it's not written swap(temp) but this seems to be common.
The other uses for this (as I thought when I read the summary and half the question... .), disregarding (bad) naming disambiguation in other answers, are if you want to cast the current object, bind it in a function object or use it with a pointer-to-member.
Casts
void Foo::bar() {
misc_nonconst_stuff();
const Foo* const_this = this;
const_this->bar(); // calls const version
dynamic_cast<Bar*>(this)->bar(); // calls specific virtual function in case of multi-inheritance
}
void Foo::bar() const {}
Binding
void Foo::baz() {
for_each(m_stuff.begin(), m_stuff.end(), bind(&Foo:framboozle, this, _1));
for_each(m_stuff.begin(), m_stuff.end(), [this](StuffUnit& s) { framboozle(s); });
}
void Foo::framboozle(StuffUnit& su) {}
std::vector<StuffUnit> m_stuff;
ptr-to-member
void Foo::boz() {
bez(&Foo::bar);
bez(&Foo::baz);
}
void Foo::bez(void (Foo::*func_ptr)()) {
for (int i=0; i<3; ++i) {
(this->*func_ptr)();
}
}
Hope it helps to show other uses of this than just this->member.
You only have to use this-> if you have a symbol with the same name in two potential namespaces. Take for example:
class A {
public:
void setMyVar(int);
void doStuff();
private:
int myVar;
}
void A::setMyVar(int myVar)
{
this->myVar = myVar; // <- Interesting point in the code
}
void A::doStuff()
{
int myVar = ::calculateSomething();
this->myVar = myVar; // <- Interesting point in the code
}
At the interesting points in the code, referring to myVar will refer to the local (parameter or variable) myVar. In order to access the class member also called myVar, you need to explicitly use "this->".
You need to use this to disambiguate between a parameters/local variables and member variables.
class Foo
{
protected:
int myX;
public:
Foo(int myX)
{
this->myX = myX;
}
};
The main (or I can say, the only) purpose of this pointer is that it points to the object used to invoke a member function.
Base on this purpose, we can have some cases that only using this pointer can solve the problem.
For example, we have to return the invoking object in a member function with argument is an same class object:
class human {
...
human & human::compare(human & h){
if (condition)
return h; // argument object
else
return *this; // invoking object
}
};
I found another interesting case of explicit usage of the "this" pointer in the Effective C++ book.
For example, say you have a const function like
unsigned String::length() const
You don't want to calculate String's length for each call, hence you want to cache it doing something like
unsigned String::length() const
{
if(!lengthInitialized)
{
length = strlen(data);
lengthInitialized = 1;
}
}
But this won't compile - you are changing the object in a const function.
The trick to solve this requires casting this to a non-const this:
String* const nonConstThis = (String* const) this;
Then, you'll be able to do in above
nonConstThis->lengthInitialized = 1;
The following code
#include <vector>
#include <string>
#include <iostream>
std::string const& at(std::vector<std::string> const& n, int i)
{
return n[i];
}
std::vector<std::string> mkvec()
{
std::vector<std::string> n;
n.push_back("kagami");
n.push_back("misao");
return n;
}
int main()
{
std::string const& s = at(mkvec(), 0);
std::cout << s << std::endl; // D'oh!
return 0;
}
may lead to crash because the original vector is already destructed there. In C++ 2011 (c++0x) after rvalue-reference is introduced in, a deleted function declaration can be used to completely forbid calls to at if the vector argument is an rvalue
std::string const& at(std::vector<std::string>&&, int) = delete;
That looks good, but the following code still cause crash
int main()
{
std::string const& s = mkvec()[0];
std::cout << s << std::endl; // D'oh!
return 0;
}
because calls to member function operator [] (size_type) const of an rvalue object is still allowed. Is there any way can I forbid this kind of calls?
FIX:
The examples above is not what I did in real projects. I just wonder if C++ 2011 support any member function qualifying like
class A {
void func() rvalue; // Then a call on an rvalue object goes to this overload
void func() const;
};
FIX:
It's great, but I think C++ standard goes too far at this feature. Anyway, I have following code compiled on clang++ 2.9
#include <cstdio>
struct A {
A() {}
void func() &
{
puts("a");
}
void func() &&
{
puts("b");
}
void func() const &
{
puts("c");
}
};
int main()
{
A().func();
A a;
a.func();
A const b;
b.func();
return 0;
}
Thanks a lot!
No, and you shouldn't. How am I to do std::cout << at(mkvec(), 0) << std::endl;, a perfectly reasonable thing, if you've banned me from using at() on temporaries?
Storing references to temporaries is just a problem C++ programmers have to deal with, unfortunately.
To answer your new question, yes, you can do this:
class A {
void func() &; // lvalues go to this one
void func() &&; // rvalues go to this one
};
A a;
a.func(); // first overload
A().func(); // second overload
Just an idea:
To disable copying constructor on the vector somehow.
vector ( const vector<T,Allocator>& x );
Implicit copying of arrays is not that good thing anyway. (wondering why STL authors decided to define such ctor at all)
It will fix problems like you've mentioned and as a bonus will force you to use more effective version of your function:
void mkvec(std::vector<std::string>& n)
{
n.push_back("kagami");
n.push_back("misao");
}
When should I explicitly write this->member in a method of
a class?
Usually, you do not have to, this-> is implied.
Sometimes, there is a name ambiguity, where it can be used to disambiguate class members and local variables. However, here is a completely different case where this-> is explicitly required.
Consider the following code:
template<class T>
struct A {
T i;
};
template<class T>
struct B : A<T> {
T foo() {
return this->i; //standard accepted by all compilers
//return i; //clang and gcc will fail
//clang 13.1.6: use of undeclared identifier 'i'
//gcc 11.3.0: 'i' was not declared in this scope
//Microsoft C++ Compiler 2019 will accept it
}
};
int main() {
B<int> b;
b.foo();
}
If you omit this->, some compilers do not know how to treat i. In order to tell it that i is indeed a member of A<T>, for any T, the this-> prefix is required.
Note: it is possible to still omit this-> prefix by using:
template<class T>
struct B : A<T> {
int foo() {
return A<T>::i; // explicitly refer to a variable in the base class
//where 'i' is now known to exist
}
};
If you declare a local variable in a method with the same name as an existing member, you will have to use this->var to access the class member instead of the local variable.
#include <iostream>
using namespace std;
class A
{
public:
int a;
void f() {
a = 4;
int a = 5;
cout << a << endl;
cout << this->a << endl;
}
};
int main()
{
A a;
a.f();
}
prints:
5
4
There are several reasons why you might need to use this pointer explicitly.
When you want to pass a reference to your object to some function.
When there is a locally declared object with the same name as the member object.
When you're trying to access members of dependent base classes.
Some people prefer the notation to visually disambiguate member accesses in their code.
Although I usually don't particular like it, I've seen others use this-> simply to get help from intellisense!
There are few cases where using this must be used, and there are others where using the this pointer is one way to solve a problem.
1) Alternatives Available: To resolve ambiguity between local variables and class members, as illustrated by #ASk.
2) No Alternative: To return a pointer or reference to this from a member function. This is frequently done (and should be done) when overloading operator+, operator-, operator=, etc:
class Foo
{
Foo& operator=(const Foo& rhs)
{
return * this;
}
};
Doing this permits an idiom known as "method chaining", where you perform several operations on an object in one line of code. Such as:
Student st;
st.SetAge (21).SetGender (male).SetClass ("C++ 101");
Some consider this consise, others consider it an abomination. Count me in the latter group.
3) No Alternative: To resolve names in dependant types. This comes up when using templates, as in this example:
#include <iostream>
template <typename Val>
class ValHolder
{
private:
Val mVal;
public:
ValHolder (const Val& val)
:
mVal (val)
{
}
Val& GetVal() { return mVal; }
};
template <typename Val>
class ValProcessor
:
public ValHolder <Val>
{
public:
ValProcessor (const Val& val)
:
ValHolder <Val> (val)
{
}
Val ComputeValue()
{
// int ret = 2 * GetVal(); // ERROR: No member 'GetVal'
int ret = 4 * this->GetVal(); // OK -- this tells compiler to examine dependant type (ValHolder)
return ret;
}
};
int main()
{
ValProcessor <int> proc (42);
const int val = proc.ComputeValue();
std::cout << val << "\n";
}
4) Alternatives Available: As a part of coding style, to document which variables are member variables as opposed to local variables. I prefer a different naming scheme where member varibales can never have the same name as locals. Currently I'm using mName for members and name for locals.
Where a member variable would be hidden by
a local variable
If you just want
to make it explictly clear that you
are calling an instance method/variable
Some coding standards use approach (2) as they claim it makes the code easier to read.
Example:
Assume MyClass has a member variable called 'count'
void MyClass::DoSomeStuff(void)
{
int count = 0;
.....
count++;
this->count = count;
}
One other case is when invoking operators. E.g. instead of
bool Type::operator!=(const Type& rhs)
{
return !operator==(rhs);
}
you can say
bool Type::operator!=(const Type& rhs)
{
return !(*this == rhs);
}
Which might be more readable. Another example is the copy-and-swap:
Type& Type::operator=(const Type& rhs)
{
Type temp(rhs);
temp.swap(*this);
}
I don't know why it's not written swap(temp) but this seems to be common.
The other uses for this (as I thought when I read the summary and half the question... .), disregarding (bad) naming disambiguation in other answers, are if you want to cast the current object, bind it in a function object or use it with a pointer-to-member.
Casts
void Foo::bar() {
misc_nonconst_stuff();
const Foo* const_this = this;
const_this->bar(); // calls const version
dynamic_cast<Bar*>(this)->bar(); // calls specific virtual function in case of multi-inheritance
}
void Foo::bar() const {}
Binding
void Foo::baz() {
for_each(m_stuff.begin(), m_stuff.end(), bind(&Foo:framboozle, this, _1));
for_each(m_stuff.begin(), m_stuff.end(), [this](StuffUnit& s) { framboozle(s); });
}
void Foo::framboozle(StuffUnit& su) {}
std::vector<StuffUnit> m_stuff;
ptr-to-member
void Foo::boz() {
bez(&Foo::bar);
bez(&Foo::baz);
}
void Foo::bez(void (Foo::*func_ptr)()) {
for (int i=0; i<3; ++i) {
(this->*func_ptr)();
}
}
Hope it helps to show other uses of this than just this->member.
You only have to use this-> if you have a symbol with the same name in two potential namespaces. Take for example:
class A {
public:
void setMyVar(int);
void doStuff();
private:
int myVar;
}
void A::setMyVar(int myVar)
{
this->myVar = myVar; // <- Interesting point in the code
}
void A::doStuff()
{
int myVar = ::calculateSomething();
this->myVar = myVar; // <- Interesting point in the code
}
At the interesting points in the code, referring to myVar will refer to the local (parameter or variable) myVar. In order to access the class member also called myVar, you need to explicitly use "this->".
You need to use this to disambiguate between a parameters/local variables and member variables.
class Foo
{
protected:
int myX;
public:
Foo(int myX)
{
this->myX = myX;
}
};
The main (or I can say, the only) purpose of this pointer is that it points to the object used to invoke a member function.
Base on this purpose, we can have some cases that only using this pointer can solve the problem.
For example, we have to return the invoking object in a member function with argument is an same class object:
class human {
...
human & human::compare(human & h){
if (condition)
return h; // argument object
else
return *this; // invoking object
}
};
I found another interesting case of explicit usage of the "this" pointer in the Effective C++ book.
For example, say you have a const function like
unsigned String::length() const
You don't want to calculate String's length for each call, hence you want to cache it doing something like
unsigned String::length() const
{
if(!lengthInitialized)
{
length = strlen(data);
lengthInitialized = 1;
}
}
But this won't compile - you are changing the object in a const function.
The trick to solve this requires casting this to a non-const this:
String* const nonConstThis = (String* const) this;
Then, you'll be able to do in above
nonConstThis->lengthInitialized = 1;