Lifetime of a temporary passed in operator[](const std::string& k)

[Daniel]

Is this code safe?

class A
{
public:
class Proxy
{
public:
friend class A;

operator A&()
{
return val_.get(k_);
}

operator const A&() const
{
return val_.get(k_);
}
private:
Proxy& operator = (const Proxy& other) {/* do nothing */}
Proxy(const Proxy& proxy)
: val_(proxy.val_), k_(proxy.k_)
{
}
Proxy(A& val, const std::string& k)
: val_(val), k_(k)
{
}

A& val_;
const std::string& k_;
};

A& get(const std::string& k)
{
// In real code, looks up another A instance using k
return *this;
}

const A& get(const std::string& k) const
{
// In real code, looks up another A instance using k
return *this;
}

Proxy operator[](const std::string& k)
{
return Proxy(*this,k);
}
};

A a;
A b = a["key"];

Do I have to worry about the lifetime of the temporary k_ held in Proxy?

Thanks!
Daniel

 

[Victor Bazarov]

Is this code safe?

class A
{
public:
class Proxy
{
public:
friend class A;

operator A&()
{
return val_.get(k_);
}

operator const A&() const
{
return val_.get(k_);
}
private:
Proxy& operator = (const Proxy& other) {/* do nothing */}
Proxy(const Proxy& proxy)
: val_(proxy.val_), k_(proxy.k_)
{
}
Proxy(A& val, const std::string& k)
: val_(val), k_(k)
{
}

A& val_;
const std::string& k_;
};

A& get(const std::string& k)
{
// In real code, looks up another A instance using k
return *this;
}

const A& get(const std::string& k) const
{
// In real code, looks up another A instance using k
return *this;
}

Proxy operator[](const std::string& k)
{
return Proxy(*this,k);
}
};

A a;
A b = a["key"];

Do I have to worry about the lifetime of the temporary k_ held in Proxy?

Unless somehow (and you don't show all the use of A:roxy an object A
can have) the object 'b' retains the reference to the string, there is
no need to worry. The temporary string created to be passed as the
argument to the operator[] has the lifetime until the end of the object
'b' initialization.

V

 

[Daniel]

Do I have to worry about the lifetime of the temporary k_ held in Proxy?

Unless somehow (and you don't show all the use of A:roxy an object A

can have) the object 'b' retains the reference to the string, there is

no need to worry. The temporary string created to be passed as the

argument to the operator[] has the lifetime until the end of the object

'b' initialization.

Thanks! Appreciate the response.

Best regards,
Daniel