C++: Problem with array of strings old C-style (as Char**)

Question

Good morning friends,

I am currently working in a big chunk of code (CFD solver) as part of a collaborative development. I am not an experienced developer and therefore the question may seem stupid, but I have come with a problem that I am unable to solve. I am trying to modify an array of old-style C-Strings (char**).

In the following part I will write the important parts of the code referring to the problem. I can not write the whole code (since it is an enormous one (millions of lines), but it should not be a problem in understanding the underlying problem). The problem arises in the last part of the written code.

PV->noVariables and m_noSpecies are ints declared before. MInt and MChar are user defined data types.

const MChar** m_variablesName;

(...)

m_variablesName = new const MChar*[PV->noVariables];
for(MInt i = 0; i < PV->noVariables; ++i) {
  m_variablesName[i] =  new MChar[10];
}

MInt count = 0;
m_variablesName[count] = "u";
count++;
m_variablesName[count] = "v";
count++;
if(nDim == 3) {
  m_variablesName[count] = "w";
  count++;
}
m_variablesName[count] = "rho";
count++;
m_variablesName[count] = "p";
count++;

(... some not relevant code...)

// here lies the problem 
if(m_noSpecies > 1) {
   for (MInt s = 0; s < m_noSpecies; s++){                                                                                                                                           
     m_variablesName[count] = ("Y_" + std::to_string(s)).c_str();                                                                                                                                    
     cerr << ("Y_" + std::to_string(s)).c_str() << endl;
     count++;
   }
}

// to check the values stored in m_variablesName
for (MInt n = 0; n < PV->noVariables; ++n)
    cerr << m_variablesName[n] << endl;

(... irrelevant code ...)

The output in the console is as follows:

First chunk of output:

Y_0
Y_1
Y_2
Y_3
Y_4
Y_5
Y_6
Y_7
Y_8
Y_9
Y_10
Y_11
Y_12
Y_13
Y_14
Y_15
Y_16
Y_17

Second chunk of output:

u
v
rho
p
Y_17
Y_17
Y_17
Y_17
Y_17
Y_17
Y_17
Y_17
Y_17
Y_17
Y_17
Y_17
Y_17
Y_17
Y_17
Y_17
Y_17
Y_17

As one can notice, the Y_0 to Y_17 have all changed to Y_17, and this is not desired. If I have understood the problem correctly, I think the problem lies in that the same pointer is stored in each array entry, and therefore all entries get the same values.

However, I do not know how to solve this problem. Some help may be greatly appreciated. The array can not be changed to work with normal C++ strings due to compatibility reasons.

Thank you all for your time!

("Y_" + std::to_string(s)).c_str() takes the address of a temporary variable. If you really cannot use std::string (which correctly manages memory for you), you will need to convert the std::string into a heap-allocated char[]. Note you will have to delete[] that memory again later. — Botje
– Botje, Commented Nov 16, 2020 at 8:30

Botje · Accepted Answer · 2020-11-16 08:35:38Z

2

Cherry-picking from your question, this line

  m_variablesName[i] =  new MChar[10];

allocates space on the heap for 10 MChars and stores the pointer in m_variablesName[i].

The problematic line is

m_variablesName[count] = ("Y_" + std::to_string(s)).c_str();

which replaces that pointer with one to a temporary object, as "Y_" + std::to_string(s) is a temporary.

In this instance, you can use snprintf instead to write the proper characters into your first buffer:

snprintf(m_variablesName[count], 10, "Y_%d", s);

You can also combine both steps in one with the asprintf function, which allocates and prints in one go.

answered Nov 16, 2020 at 8:35

Botje

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4 Comments

Borja BePe Over a year ago

Thank you for your insight Botje, very much appreciated! I would never have come to this answer by myself. However, I made a slight error when writing the code (it is now correct). The initialisation of m_variablesName is as "const MChar** m_variablesName; ". When I try your suggested snprintf-function, the compiler thrwos an error due to the const definition. Is there a workaround/ does this suppose an additional problem?

Botje Over a year ago

Then you have to choose. Either make it non const by default, temporarily const_cast, or replace the pointer with a heap-allocated char[].

Borja BePe Over a year ago

Thank you again Botje! I ended up using the heap allocated char[], since the constant nature was needed due to compatibility reasons with other function calls. Now it works perfectly!

Botje Over a year ago

Don't forget to delete[] that memory.

rustyhu · Accepted Answer · 2020-11-16 10:55:14Z

0

All m_variablesName[count] in this snippet are allocated by

for (MInt i = 0; i < PV->noVariables; ++i) {
  m_variablesName[i] =  new MChar[10];
}

These C-style strings can not be assign directly, you should use functions such as strncpy, to copy contents into:

for (MInt s = 0; s < m_noSpecies; s++) {
    memset(m_variablesName[count], 0, 10);
    std::string strtmp = "Y_" + std::to_string(s);
    strncpy(m_variablesName[count], strtmp.c_str(), 9);
    count++;
}

answered Nov 16, 2020 at 10:55

rustyhu

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1 Comment

Borja BePe Over a year ago

Thank you for your answer! I ended using a similar piece of code as to what you suggested, and it worked perfectly!

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