How to printf to std::string by using of a function?

Question

Hi, I want to printf to std::string by using of a function, but I cannot do it.

EDIT from answer: Use vsnprintf instead of snprintf.

#include <string> // std::string
using std::string;
#include <stdarg.h> // va_list, va_start, va_arg, va_end

#define EMC_VERIFY(x) // TODO: Replace this.
#define SAFE_FREE(p) { if (p) { free (p); (p)=nullptr; } } ((void)0) // There must be ";".

string string_sprintf(const char *format, ...) {
    va_list args;
    va_start(args, format);
        // Referenced from https://stackoverflow.com/questions/436367/best-way-to-safely-printf-to-a-string
        size_t sz = vsnprintf(nullptr, 0, format, args);
        size_t bufsize = sz + 1;
        char *buf = (char *)malloc(bufsize);
        EMC_VERIFY(buf != nullptr);
        vsnprintf(buf, bufsize, format, args);
        //buf[bufsize - 1] = '\0'; // This line is not necessary, check the official documentation of vsnprintf for proof.
    va_end(args);
    string str = buf;
    SAFE_FREE(buf);
    return str;
}
int main()
{
    string s = string_sprintf("Hello World %d %d", 11, 22);
    return 0;
}

Wrong Result when using of snprintf:
s = "Hello World 7862840 7863104"
Why it doesn't work? And how to fix it?

Fixed Result when using of vsnprintf:
s = "Hello World 11 22"

Answer 1

Hi @Manda Rajo ,

I suggest you try this sample:

#include <cstdio>  
#include <string>  
#include <cassert>  
#include<iostream>  
template< typename... Args >  
std::string string_sprintf(const char* format, Args... args) {  
	int length = std::snprintf(nullptr, 0, format, args...);  
	assert(length >= 0);  
  
	char* buf = new char[length + 1];  
	std::snprintf(buf, length + 1, format, args...);  
  
	std::string str(buf);  
	delete[] buf;  
	return str;  
}  
template<typename ... Args>  
std::string string_format(const std::string& format, Args ... args)  
{  
	int size_s = std::snprintf(nullptr, 0, format.c_str(), args ...) + 1; // Extra space for '\0'  
	if (size_s <= 0) { throw std::runtime_error("Error during formatting."); }  
	auto size = static_cast<size_t>(size_s);  
	std::unique_ptr<char[]> buf(new char[size]);  
	std::snprintf(buf.get(), size, format.c_str(), args ...);  
	return std::string(buf.get(), buf.get() + size - 1); // We don't want the '\0' inside  
}  
int main()  
{  
	std::cout << string_sprintf("sdad\n");  
	std::cout << string_sprintf("%g, %g\n", 1.23, 0.001);  
	  
	return 0;  
}  

Best regards,

Elya

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Answer 2

I cannot place that function in a common project, because if I make it inline in a header file in the common project then another error because EMC_VERIFY is cross-platform such as on Windows, it requires MessageBox and MB_OK, and the most difficult is it requires "Windows.h" which is the most difficult to place, because I must always place it as last include.
So I don't make it inline by declaring in a header file and put the definition in a .cpp file as below, and I got errors, I'm sure it's because I'm using a template.

Location: Project "emc-common" > "emc-common_internal.h"

   template<typename... Args>  
   string string_sprintf(const char *format, Args... args);  

Location: Project "emc-common" > "emc-common_internal.cpp"

   template<typename... Args>  
   string string_sprintf(const char *format, Args... args) {  
       // Originated from https://learn.microsoft.com/en-us/answers/questions/813614/how-to-printf-to-stdstring-by-using-of-a-function.html  
       int length = std::snprintf(nullptr, 0, format, args...);  
       EMC_VERIFY(length >= 0);  

       size_t sz = length + 1; // Extra space for '\0'  
       char *buf = (char *)malloc(sz);  
       std::snprintf(buf, sz, format, args...);  

       string str = buf;  
       SAFE_FREE(buf);  
       return str;  
   }  

Location: Project "emc-language" > "emc-language.cpp"

   string g_output = "";  
   #define print(...) \  
       do { \  
           g_output += string_sprintf(__VA_ARGS__); \  
       } while(0)  

Error:

`

emc-language_debug.lib(emc-language.obj) : error LNK2019: unresolved external symbol "class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl string_sprintf<char const *>(char const *,char const *)" (??$string_sprintf@PEBD@@YA?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@PEBD0@Z) referenced in function "void __cdecl printBuiltin(struct emcl::astBuiltin *)" (?printBuiltin@@YAXPEAUastBuiltin@emcl@@@Z)

`

`

emc-language_debug.lib(emc-language.obj) : error LNK2019: unresolved external symbol "class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl string_sprintf<char *>(char const *,char *)" (??$string_sprintf@PEAD@@YA?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@PEBDPEAD@Z) referenced in function "void __cdecl printType(struct emcl::astType *)" (?printType@@YAXPEAUastType@emcl@@@Z)

`

`

emc-language_debug.lib(emc-language.obj) : error LNK2019: unresolved external symbol "class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl string_sprintf<int>(char const *,int)" (??$string_sprintf@H@@YA?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@PEBDH@Z) referenced in function "void __cdecl printIntConstant(struct emcl::astIntConstant *)" (?printIntConstant@@YAXPEAUastIntConstant@emcl@@@Z)

`

`

emc-language_debug.lib(emc-language.obj) : error LNK2019: unresolved external symbol "class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl string_sprintf<unsigned int>(char const *,unsigned int)" (??$string_sprintf@I@@YA?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@PEBDI@Z) referenced in function "void __cdecl printUIntConstant(struct emcl::astUIntConstant *)" (?printUIntConstant@@YAXPEAUastUIntConstant@emcl@@@Z)

`

`

emc-language_debug.lib(emc-language.obj) : error LNK2019: unresolved external symbol "class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl string_sprintf<float>(char const *,float)" (??$string_sprintf@M@@YA?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@PEBDM@Z) referenced in function "void __cdecl printFloatConstant(struct emcl::astFloatConstant *)" (?printFloatConstant@@YAXPEAUastFloatConstant@emcl@@@Z)

`

`

emc-language_debug.lib(emc-language.obj) : error LNK2019: unresolved external symbol "class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl string_sprintf<double>(char const *,double)" (??$string_sprintf@N@@YA?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@PEBDN@Z) referenced in function "void __cdecl printDoubleConstant(struct emcl::astDoubleConstant *)" (?printDoubleConstant@@YAXPEAUastDoubleConstant@emcl@@@Z)

`

`

emc-language_debug.lib(emc-language.obj) : error LNK2019: unresolved external symbol "class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl string_sprintf<>(char const *)" (??$string_sprintf@$$V@@YA?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@PEBD@Z) referenced in function "void __cdecl printStatement(struct emcl::astStatement *)" (?printStatement@@YAXPEAUastStatement@emcl@@@Z)

`

Answer 3

The solution for that error is to use string string_sprintf(const char *format, ...); from the first post because it doesn't use tamplate, and I did it and there is no error, I made a big EDIT on the first post by inspiring from all the answers, so "Thank you very much".