Question about 'this' in C++

lowpro2k3 · 08-21-2005, 08:29 PM

When programming a class in C++, I'm wondering if there is any speed difference or other negative reasons for using the 'this' pointer to access data members. Its harder for me to explain, but easier to show with code. I'm wondering if these 2 class methods have any difference from eachother.

Code:

class Test
{
private:
    string data_;

public:
    void setData1(string& data);
    void setData2(string& data);

};

void Test::setData1(string& data)
{
    // use 'this' for extra clarity
    this->data_ = data;
}

void Test::setData2(string& data)
{
    // don't use 'this'
    data_ = data;
}

I prefer using the 'this->data_member' notation because it makes my classes easier to read. I'm wondering if there is any negative impact to using this notation.

I was also wondering the same thing about the resolution operator, '::'. I can't see any negative effects of using it in classes like this:

Code:

// same class definition as above

void Test::setData1(string& data)
{
    // Use scope resolution operator
    Test::data_ = data;
}

void Test::setData2(string& data)
{
    // Dont use scope resolution operator
    data_ = data;
}

FreeThinkerJim · 08-21-2005, 10:42 PM

Quote:

Originally posted by lowpro2k3
When programming a class in C++, I'm wondering if there is any speed difference or other negative reasons for using the 'this' pointer to access data members. Its harder for me to explain, but easier to show with code. I'm wondering if these 2 class methods have any difference from eachother.

Code:

class Test { private: string data_; public: void setData1(string& data); void setData2(string& data); }; void Test::setData1(string& data) { // use 'this' for extra clarity this->data_ = data; } void Test::setData2(string& data) { // don't use 'this' data_ = data; }

I prefer using the 'this->data_member' notation because it makes my classes easier to read. I'm wondering if there is any negative impact to using this notation.

Using the 'this' pointer to reference a class's data within its methods is totally superfluous. When the compiler writes out the machine code for a method, it does this anyway, and so putting it in your code doesn't really have an effect.

Quote:

I was also wondering the same thing about the resolution operator, '::'. I can't see any negative effects of using it in classes like this:

Code:

// same class definition as above void Test::setData1(string& data) { // Use scope resolution operator Test::data_ = data; } void Test::setData2(string& data) { // Dont use scope resolution operator data_ = data; }

Like the this pointer thing, doing this is also unnecessary. Doing these things might make your code seem more readable, but it's really nonstandard and can throw a lot of people off

carl.waldbieser · 08-22-2005, 12:13 AM

Quote:

Originally posted by lowpro2k3

I was also wondering the same thing about the resolution operator, '::'. I can't see any negative effects of using it in classes like this:

Code:

// same class definition as above void Test::setData1(string& data) { // Use scope resolution operator Test::data_ = data; } void Test::setData2(string& data) { // Dont use scope resolution operator data_ = data; }

Using the resolution operator is not a problem in this case. However, it could lead to problems if you want to use it in a polymorphic class. If you explicitly qualify a member, it is going to use the member of the class you specify no matter what. Sometimes this is the behavior you want. Sometimes you would rather have a derived class's member be referenced.