Concatenate strings in function call c++

Question

I'm trying to create a logger function where you can pass in a message which will be logged to a text file. Sometimes I'd like to pass in a variable concatenated with my message so I could do something like:

logger("The variable is: " + variable);

The function is defined as

void logger(std::string message);

I'm using Qt, so I don't know if it's relevant but the variable will always be a QString. When I tried this it would say that no candidate function for

void logger(const QString);

So I thought why not make a second function where it would expect a concatenation:

void logger(std::string message);
void logger2(const QString message);

It compiled fine when I did

logger2("The variable is: " + variable);

However, when I debugged the passed message variable was an empty string. How do I get this to work, is it possible?

variable is almost always a QString. I would have done const QString& but the error message said I was doing const QString. Having const QString& doesn't work as "Text" + QString == const QString. If I do const QString& it doesn't compile — robev
– robev, Commented Mar 22, 2011 at 19:42

John Dibling · Accepted Answer · 2011-03-22 19:45:57Z

6

Why not just do something like this:

QString qs = "hello";
qs.toStdString();

As far as concatenating on the fly, I like to use a simple formatter class:

class Formatter
{
public:
    template<class Val> Formatter& operator<<(const Val& val)
    {
        ss_ << val;
        return * this;
    }
    operator string () const { return ss_.str().c_str(); }
private:
    std::stringstream ss_;
};

...which can be used like this:

logger(Formatter() << "The variable is: " << variable);

answered Mar 22, 2011 at 19:45

John Dibling

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1 Comment

John Dibling Over a year ago

BTW, you can see a code review of my Formatter class here: codereview.stackexchange.com/questions/226/…

mPopp · Accepted Answer · 2011-03-23 12:24:42Z

0

have you tried logger("The variable is: " + variable.toStdString()); ?

answered Mar 23, 2011 at 12:24

mPopp

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1 Comment

mPopp Over a year ago

Sorry for the late comment. The QString include should do it.

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