如何正确地将C字符串传递给可变参数模板函数内的lambda

Question

Here is my complex situation:

I have a function using variadic template and lambda:

template<typename...Args>
void foo(Args...args) {
    // the arguments are passed to the lambda
    _func = [args...](){ do_sth(args...); };
}

On observing specific events, the lambda _func would be fired. My problem is, some of the arguments I passed are C strings, they could be pointing to temporary std string like this:

const char *pStr = __temp_std__string__.c_str();

Given I call foo(pStr); , and when _func is called, the temporary string that pStr is pointing to, has been released. I would like to know whether there exists a generic way to handle this. I am using C++11.

EDIT:

Perhaps I should post my whole story, as many of you advise to pass std::string instead of C strings, there are reasons that I can't escape from it.

I am developing games using cocos2d-x, which deploys C++11. What I want to do is to support auto-localisation of labels when players change their preferences of languages (selected from a UI).

I have saved the text in a couple of files, and each of them contains the localised text of a single language, they are basically under the following structure:

{
    "key1" : "_localized_text1_",
    "key2" : "_localized_text2_",
    ...
}

The idea is to observe the event on change of language's preference (through a notification), and I would get a key indicating that language from it, so as to fetch the localised text from the proper file. Here is the way how I implement it in the object class Label :

class Label {
    // this method would update the label's displayed text
    void setString(const std::string& text);

    // set a callback for changing language
    void setOnLanguageChangeFunc(std::function<void(Notification*)> func);

    // set a localised text, which would be updated on changing language
    void setLocalizeString(const std::string& key);
};

the core function is setLocalizeString (I skip the implementations of the other 2 methods as they are intuitive enough from their declaration):

void Label::setLocalizeString(const std::string& key) {
    // the callback lambda
    auto callback = [=](Notification *pNotification){
      setString(LOCALIZED_STRING(key)); 
    }

    // assign the lambda
    setOnLanguageChangeFunc(callback);
}

where LOCALIZED_STRING is the macro helper of fetching localised string with a key; and the lambda callback would be saved as local member variable of Label in setOnLanguageChangeFunc .

This works great in most cases, what makes the situation complicated is, there are format specifiers involved in the localised text, for example:

{
    ...
    "keyN" : "%s eats %d cookies",
    ...
}

Such format placeholders are passed dynamically in codes:

// formatStr = "Tom eats 5 cookies"
std::string formatStr = StringKit::stringWithFormat("%s eats %d cookies", "Tom", 5);

where StringKit is a utility to format the string, and it accepts variadic arguments which would be passed to vsnprintf to yield the output. Now you know why I need to pass C string and not std::string, its just due to the underlying method to format string.

Now I have to modify Label::setLocalizeString so that it could digest the possible variadic arguments:

template<typename... Args>
void setLocalizeString(const std::string& key, Args... args)
{
    // the callback lambda
    auto callback = [=](Notification *pNotification){
         setString(StringKit::stringWithFormat(LOCALIZED_STRING(sKey), args...)); 
    }

    // assign the lambda
    setOnLanguageChangeFunc(callback);
}

And this is its use case:

// on changing language, the label would display "Tom eats 5 cookies"
pLabel->setLocalizeString("keyN", "Tom", 5);

This case would work like a charm as that C string argument is global, but when it is passed from a temporary std::string :

std::string tempStr = "Tom";
pLabel->setLocalizeString("keyN", tempStr.c_str(), 5);

The C string "Tom" would lose the value on calling the lambda callback, since the pointed std::string, has been gone.

I have tried several ways, like playing with tuple things, or capturing a wrapper class of basic types in the lambda, but none of them could solve the problem. However, I think there should exist some tricky solutions.

Answer 1

This problem is not related to lambdas or variadic functions - it also occurs if you simply store the string:

const char* global_storage;

int main()
{
    {
        std::string s = "hi";
        global_storage = s.c_str();
    }

    // !!! `global_storage` points to deleted memory!
    use(global_storage);
}

You need to make sure the string lives long enough. Using std::string instead of const char* is a great starting point:

std::string global_storage;

int main()
{
    {
        std::string s = "hi";
        global_storage = std::move(s);
    }

    // OK, local string was moved into `global_storage`.
    use(global_storage.c_str());
}

If you really need to use a C-style string, just store it in the lambda/whatever as a std::string , then call .c_str() when you need to use it, not when storing it.

Answer 2

You need to convert your char const* arguments to std::string when storing it in lambda. This is one possible way, i can propose:

#include <iostream>
#include <tuple>
using namespace std;

template<typename T, typename R = conditional_t<is_same<T, char const*>::value, string, T>>
R bar (T &&value) {return value;}

template<class Ch, class Tr, class Tuple, std::size_t... Is>
void print_tuple_impl(std::basic_ostream<Ch,Tr>& os,
                      const Tuple & t,
                      std::index_sequence<Is...>)
{
    using swallow = int[]; // guaranties left to right order
    (void)swallow{0, (void(os << (Is == 0? "" : ", ") << std::get<Is>(t)), 0)...};
}

template<class Ch, class Tr, class... Args>
decltype(auto) operator<<(std::basic_ostream<Ch, Tr>& os,
                          const std::tuple<Args...>& t)
{
    os << "(";
    print_tuple_impl(os, t, std::index_sequence_for<Args...>{});
    return os << ")";
}

template<typename...Args>
decltype(auto) foo(Args...args)
{
    return [args = make_tuple(bar(args)...)] () { cout<< args; return; };
}

int main() {
    string *s = new string("Hello, World!");
    const char *p = s->c_str();
    auto f = foo(1, p, 3.14);
    delete s;
    f();
    return 0;
}

Function foo returns lambda that stores variadic arguments as tuple, where each char const* element is converted to std::string automatically. After that you can free temporary string. It's now should be safe to call that lambda after freeing.

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