---
title: Fixing 'to_string was not declared in this scope': Comprehensive Guide for Troubleshooting and Solutions
description: Learn the causes and solutions for 'to_string was not declared in this scope' error in C++.
---
In this comprehensive guide, we'll help you troubleshoot and resolve the common C++ error, `to_string was not declared in this scope`. This error occurs when the compiler is unable to find the `std::to_string` function, which is used to convert numeric values to strings. We'll cover the possible causes and provide step-by-step solutions to fix this issue.
## Table of Contents
- [Possible Causes](#possible-causes)
- [Solutions](#solutions)
- [Solution 1: Include the Required Header](#solution-1-include-the-required-header)
- [Solution 2: Use the Correct Namespace](#solution-2-use-the-correct-namespace)
- [Solution 3: Upgrade Your Compiler](#solution-3-upgrade-your-compiler)
- [Solution 4: Write Your Own to_string Function](#solution-4-write-your-own-to_string-function)
- [FAQs](#faqs)
## Possible Causes
There are several reasons why you might encounter the `'to_string was not declared in this scope'` error while working with C++:
1. The `<string>` header is not included in your source code.
2. The `std::` namespace is not being used correctly.
3. Your compiler might not support `std::to_string`.
4. There might be a conflict with other libraries or namespaces.
## Solutions
### Solution 1: Include the Required Header
The `std::to_string` function is declared in the `<string>` header. To resolve the error, ensure that you have included the `<string>` header in your source code.
```cpp
#include <string>
int main() {
int number = 42;
std::string str_number = std::to_string(number);
return 0;
}
Solution 2: Use the Correct Namespace
The std::to_string function belongs to the std namespace. Ensure that you're using the std:: prefix when calling to_string or include the using directive for the std namespace.
#include <string>
using namespace std;
int main() {
int number = 42;
string str_number = to_string(number);
return 0;
}
Solution 3: Upgrade Your Compiler
Some older versions of compilers, like GCC 4.7 and earlier, might not support std::to_string. To resolve this issue, consider upgrading your compiler to a newer version.
For example, to upgrade GCC on Ubuntu, run the following commands:
sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt update
sudo apt install gcc-9 g++-9
After upgrading, make sure that your new compiler is being used by default. You can do this by running gcc --version and checking the displayed version.
Solution 4: Write Your Own to_string Function
If upgrading your compiler is not an option, you can write your own to_string function using std::ostringstream. Here's an example implementation:
#include <string>
#include <sstream>
template <typename T>
std::string to_string(const T& value) {
std::ostringstream os;
os << value;
return os.str();
}
int main() {
int number = 42;
std::string str_number = to_string(number);
return 0;
}
FAQs
Q1: Can I use std::to_string with custom classes?
A: Yes, you can use std::to_string with custom classes by overloading the << operator for your class and writing your own to_string function as shown in Solution 4.
Q2: Why am I getting a 'std::to_string' has not been declared error even after including the <string> header?
A: You might be using an older compiler that doesn't support std::to_string. Try upgrading your compiler as described in Solution 3.
Q3: Can I use std::to_string with floating-point numbers?
A: Yes, std::to_string supports floating-point numbers. It has overloads for float, double, and long double.
Q4: How can I specify the number of decimal places when using std::to_string with floating-point numbers?
A: std::to_string does not provide an option to specify the number of decimal places. Instead, you can use std::ostringstream with std::setprecision to achieve this:
#include <string>
#include <sstream>
#include <iomanip>
std::string to_string_with_precision(double value, int precision) {
std::ostringstream os;
os << std::fixed << std::setprecision(precision) << value;
return os.str();
}
Q5: How can I convert a string to a number in C++?
A: You can use std::stoi for converting a string to an int, std::stol for long, std::stoll for long long, std::stof for float, std::stod for double, and std::stold for long double. These functions are declared in the <string> header.
#include <string>
int main() {
std::string str_number = "42";
int number = std::stoi(str_number);
return 0;
}