C++ Simple IF clause changes value of a static const char variable

问题

Okay, I've been dealing with this for two days now, and I can't find a solution.

Problem: I'm trying to set a filter to a File Selection Dialog using Winapi. I'm using GetOpenFileName function to do this. This function uses a structure to set options such as file extension filters. This structure's member called lpstrFilter needs a certain string format. I'm setting that string exactly as Winapi indicates, but for some reason, this string's value changes.

I've got this static const char *:

//This contains string "JPG"
static const char * extensionFilter = v->trabajo10.C_JMV_SelectFile_FileExtension7.GetString();

//This forms a filter string which applies to OPENFILENAME structure.
string sFilter;
sFilter.append("Format: ");
sFilter.append(extensionFilter);
sFilter.push_back('\0');
sFilter.append("*.");
sFilter.append(extensionFilter);
sFilter.push_back('\0');
const char * filter = sFilter.c_str();
ofn.lpstrFilter = filter; //This sets: --> Format: JPG\0*.JPG\0

//This opens the file selection dialog
if (GetOpenFileName(&ofn)==TRUE){
...

The File Selection Dialog looks CORRECTLY, like this:

The joke comes now, I modify the code like this:

//This contains string "JPG"
static const char * extensionFilter = v->trabajo10.C_JMV_SelectFile_FileExtension7.GetString();

if(1){
   //This forms a filter string which applies to OPENFILENAME structure.
   string sFilter;
   sFilter.append("Format: ");
   sFilter.append(extensionFilter);
   sFilter.push_back('\0');
   sFilter.append("*.");
   sFilter.append(extensionFilter);
   sFilter.push_back('\0');
   const char * filter = sFilter.c_str();
   ofn.lpstrFilter = filter; //This sets: --> Format: JPG\0*.JPG\0
}

//This opens the file selection dialog
if (GetOpenFileName(&ofn)==TRUE){
...

And this is the result, THE PROBLEM:

Filter string was modified???

回答1:

if(1){
   //This forms a filter string which applies to OPENFILENAME structure.
   string sFilter;
   sFilter.append("Format: ");
   sFilter.append(extensionFilter);
   sFilter.push_back('\0');
   sFilter.append("*.");
   sFilter.append(extensionFilter);
   sFilter.push_back('\0');
   const char * filter = sFilter.c_str();
   ofn.lpstrFilter = filter; //This sets: --> Format: JPG\0*.JPG\0
}

The sFilter variable has a lifetime that ends when the block in which it declared ends. The pointer returned by sFilter.c_str() is valid until sFilter is modified or destroyed.

You are using this pointer after it has become invalidated. This is the same problem as you had yesterday, which I guessed at in comments to the question. This is why you need to show a full MCVE. This question also looks to be a duplicate of the one that you asked a week ago: Winapi GetOpenFileName Extension Filter not working. I suggest that you take some time to make sure that you fully appreciate the validity of the value returned by c_str().

You must ensure that sFilter lives until after you have finished with the pointer. Declare sFilter in an outer block to ensure that.

回答2:

The problem is that you have a variable that fell out of scope

if(1){
   string sFilter;

   // ... code

   // Right here
   const char * filter = sFilter.c_str();      
   ofn.lpstrFilter = filter;
}

After that block ends filter falls out of scope, so ofn.lpstrFilter has a dangling pointer.

回答3:

Answering ProtectedVoid's concern about declaring an unused object: Imagine a std::string was an expensive object and the condition was unlikely. You don't want the object constructed unless the condition is true. But then it must last beyond the scope of the condition. So we use the fact that a default constructed unique_ptr is much cheaper than a default constructed string:

std::unique_ptr<std::string> scope_extender;
if( something unlikely ){
       //This forms a filter string which applies to OPENFILENAME structure.
       std::string* sFilter = new std::string;
       scope_extender.reset( sFilter );
       sFilter->append("Format: ");
       sFilter->append(extensionFilter);
       sFilter->push_back('\0');
       sFilter->append("*.");
       sFilter->append(extensionFilter);
       sFilter->push_back('\0');
       const char * filter = sFilter->c_str();
       ofn.lpstrFilter = filter; //This sets: --> Format: JPG\0*.JPG\0
    }

Obviously I don't want to imply std::string is expensive enough to construct to be worth all that trouble. But some object in a similar situation might be. Also, there was a comment about what if it was many minor objects in one conditional. In that case, you would want a utility struct to hold them all together and the unique_ptr conditionally owns that struct.

来源：https://stackoverflow.com/questions/34223930/c-simple-if-clause-changes-value-of-a-static-const-char-variable

标签

c++

winapi

memory

reference

getopenfilename