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Emiliano Torres
Asked: 2020-02-22 07:09:33 +0800 CST 2020-02-22 07:09:33 +0800 CST

Warning: deprecated conversion from string constant

  • 772

I have a warning that I can't remove...it tells me "deprecated conversion from string constant to char*". The code where it complains is the following:

class THREE_DATA_4_COMPUTE
{
    public:
        THREE_DATA_4_COMPUTE(const QString &cn, char* umn = "", const QString &un = (const QString)"")
        {
            this->curveName = cn;
            this->unitModuleName = umn;
            this->unitName = un;
        };
        QString curveName;
        char* unitModuleName;
        QString unitName;
    protected:
        THREE_DATA_4_COMPUTE(){};
};

and I call it as follows, passing only 1 parameter (hence I need the defaults):

THREE_DATA_4_COMPUTE *aux = new THREE_DATA_4_COMPUTE(this->getRmft());

What is wrong?. I am with c++, Qt 4.8 and linux.

c++

2 Answers

  1. Best Answer
    THREE_DATA_4_COMPUTE(const QString &cn, char* umn = "", const QString &un = (const QString)"")
//                                                  ^^                       ^^^^^^^^^^^^^^^^
//                                                   1                       2

    Those two mistakes:

    1. ""is not a string variable but a constant, just as "hola"and therefore such a string should only be pointed to from a constant pointer
    2. That structure doesn't make any sense. A non-constant item can become a constant at any time without penalty because all you do is increase the item's constraint, and that 's always allowed. On the other hand, you don't need to build the QStringfrom a string because you already have a constructor for it

    How should that constructor look like?

    THREE_DATA_4_COMPUTE(const QString &cn, const char* umn = "", const QString &un = QString())

    One problem you may run into when trying this signature is that umnit is now not compatible with unitModuleName, since one is const char*and the other char*. In my opinion, unless strictly necessary, I would convert unitModuleNameto QString... although the same goes for changing unitModuleNameto be const char*... since you don't show the implementation of the class it is not clear if that value can change or not.

    Of course, since it's free to put longer variable names, for readability I'd almost advise you to make variable names more descriptive.

    • 4

  2. Problem.

    The alarm deprecated conversion from string constant to char*is usually given when trying to store a text literal whose type is a constant pointer to char, in a variable that is a pointer to char(without being a constant).

    You may also encounter this alarm when trying to store a constant charpointer to a charnon-constant pointer to.

    Examples.

    void funcion(char *texto);

// deprecated conversion from string constant to char*
funcion("Patatas fritas, que ricas!");

    In the code above we see how a text literal ( "Patatas fritas, que ricas!") is stored in a variable of type pointer to charwithout the qualifier const. The type of the text literal is const char[27](constant array of 27 elements of type char), this type of array is implicitly convertible to a constant pointer to char; when passing it to a non-constant pointer, the alarm appears. To correct the problem add the qualifier constto the parameter:

    void funcion(const char *texto); // const anyadido
funcion("Patatas fritas, que ricas!"); // ninguna alarma

    It can also happen if the return of a function is const char *and is stored in a pointer variable to charthat does not have the qualifier const:

    struct C { const char *miembro() { return nullptr; } };
void funcion(char *texto);

C c;

// deprecated conversion from string constant to char*
funcion(c.miembro());

    Why does it happen?: Compatibility with

    The implicit conversion of const char *a char *is the only such conversion allowed in C++, it exists for backward compatibility with C.

    In the C language the semantics of the text literals and the qualifier constare different than in C++. In C a text literal is an array of charnon-constants while in C++ a text literal is an array of charconstants. To allow the same semantics in C as in C++, this conversion was marked as an alarm instead of an error.

    So in C passing a text literal to a function is allowed:

    // Si compilamos con C en lugar de con C++ ...
void funcion(char *texto);

// ... esto no da problemas ni alarmas.
funcion("Patatas fritas, que ricas!");

    However, conversions from qualified data constto unqualified data are explicitly prohibited in C++:

    // Funcion que recibe un puntero a char
void funcion(char *texto);
// Funcion que recibe un puntero a int
void funcion_int(int *números);
// Un arreglo de valores int.
const int numeros[]{0,1,2,3,4,5,6,7,8,9};

// deprecated conversion from string constant to char*
funcion("Patatas fritas, que ricas!");
// Error! NO se puede convertir 'const int *' a 'int *'.
funcion_int(numeros);

    While with we get the deprecatedchar alarm with we get an error. If you use newer C++ compilers you will get an error instead of an alarm.int

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